Begin work for supporting GoogleSignIn server-side

I'm attempting to be an obedient boy and implement this and future features
using TDD.

TL;DR:
- Defined a few tests
- Defined an empty GoogleSignIn module
- Defined a Fixtures module to quickly create JWTs to test

website/sandbox/learnpianochords/src/server/Fixtures.hs

@@ -0,0 +1,53 @@
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE RecordWildCards #-}
+--------------------------------------------------------------------------------
+module Fixtures where
+--------------------------------------------------------------------------------
+import Web.JWT
+import Utils
+
+import qualified Data.Map as Map
+--------------------------------------------------------------------------------
+
+-- | These are the JWT fields that I'd like to overwrite in the `googleJWT`
+-- function.
+data JWTFields = JWTFields
+  { overwriteSigner :: Signer
+  , overwriteAud :: Maybe StringOrURI
+  }
+
+defaultJWTFields :: JWTFields
+defaultJWTFields = JWTFields
+  { overwriteSigner = hmacSecret "secret"
+  , overwriteAud = stringOrURI "771151720060-buofllhed98fgt0j22locma05e7rpngl.apps.googleusercontent.com"
+  }
+
+googleJWT :: JWTFields -> Maybe (JWT UnverifiedJWT)
+googleJWT JWTFields{..} =
+  encodeSigned signer jwtHeader claimSet
+  |> decode
+  where
+    signer :: Signer
+    signer = overwriteSigner
+
+    jwtHeader :: JOSEHeader
+    jwtHeader = JOSEHeader
+      { typ = Just "JWT"
+      , cty = Nothing
+      , alg = Just RS256
+      , kid = Just "f05415b13acb9590f70df862765c655f5a7a019e"
+      }
+
+    claimSet :: JWTClaimsSet
+    claimSet = JWTClaimsSet
+      { iss = stringOrURI "accounts.google.com"
+      , sub = stringOrURI "114079822315085727057"
+      , aud = overwriteAud |> fmap Left
+      -- TODO: Replace date creation with a human-readable date constructor.
+      , Web.JWT.exp = numericDate 1596756453
+      , nbf = Nothing
+      -- TODO: Replace date creation with a human-readable date constructor.
+      , iat = numericDate 1596752853
+      , unregisteredClaims = ClaimsMap (Map.fromList [])
+      , jti = stringOrURI "0d3d7fa1fe05bedec0a91c88294936b2b4d1b13c"
+      }