Support for multiproject builds with nix

- Closes #28

common/LICENSE

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+Copyright (c) 2018, Mats Rauhala
+
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+
+    * Redistributions in binary form must reproduce the above
+      copyright notice, this list of conditions and the following
+      disclaimer in the documentation and/or other materials provided
+      with the distribution.
+
+    * Neither the name of Mats Rauhala nor the names of other
+      contributors may be used to endorse or promote products derived
+      from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

common/common.cabal

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+name:                common
+version:             0.1.0.0
+-- synopsis:
+-- description:
+license:             BSD3
+license-file:        LICENSE
+author:              Mats Rauhala
+maintainer:          mats.rauhala@iki.fi
+-- copyright:
+category:            Web
+build-type:          Simple
+extra-source-files:  ChangeLog.md
+cabal-version:       >=1.10
+
+library
+  exposed-modules:     Configuration
+                     , Data.Versioned
+  -- other-extensions:
+  build-depends:       base >=4.10 && <4.11
+                     , aeson
+                     , asn1-data
+                     , asn1-types
+                     , bytestring
+                     , classy-prelude
+                     , cryptonite
+                     , dhall
+                     , directory
+                     , foreign-store
+                     , generic-lens
+                     , http-api-data
+                     , http-media
+                     , jose
+                     , lens
+                     , lucid
+                     , memory
+                     , monad-logger
+                     , mtl
+                     , pandoc
+                     , pandoc-types
+                     , pem
+                     , process
+                     , resource-pool
+                     , selda
+                     , selda-postgresql
+                     , servant
+                     , servant-auth
+                     , servant-auth-server
+                     , servant-docs
+                     , servant-lucid
+                     , servant-multipart
+                     , servant-server
+                     , text
+                     , transformers
+                     , wai
+                     , warp
+                     , x509
+                     , x509-store
+                     , xml-conduit
+                     , xml-hamlet
+  hs-source-dirs:      src
+  default-extensions:  DeriveGeneric
+                     , NoImplicitPrelude
+                     , OverloadedStrings
+                     , RecordWildCards
+  default-language:    Haskell2010

common/src/Configuration.hs

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+{-# Language NoImplicitPrelude #-}
+{-# Language DeriveGeneric #-}
+{-# Language DuplicateRecordFields #-}
+module Configuration where
+
+import ClassyPrelude
+import Dhall (Interpret)
+
+data Pg = Pg { username :: Text
+             , password :: Text
+             , host :: Text
+             , database :: Text }
+        deriving (Show, Generic)
+
+newtype Store = Store { path :: Text } deriving (Show, Generic)
+
+data Config = Config { database :: Pg
+                     , store :: Store }
+            deriving (Show, Generic)
+
+instance Interpret Pg
+instance Interpret Store
+instance Interpret Config

common/src/Data/Versioned.hs

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+{-# Language KindSignatures #-}
+{-# Language DataKinds #-}
+{-# Language DefaultSignatures #-}
+{-# Language MultiParamTypeClasses #-}
+{-# Language FunctionalDependencies #-}
+module Data.Versioned where
+
+import GHC.TypeLits
+import ClassyPrelude
+import Data.Generics.Product
+
+newtype Versioned (v :: Nat) a = Versioned a deriving (Show)
+
+instance Functor (Versioned v) where
+  fmap f (Versioned a) = Versioned (f a)
+
+instance Applicative (Versioned v) where
+  pure = Versioned
+  (Versioned f) <*> (Versioned a) = Versioned (f a)
+
+class Migrate a b | b -> a where
+  migrate :: a -> b
+  default migrate :: (Subtype b a) => a -> b
+  migrate = upcast