Verify users' email addresses when they attempt to sign-up

Lots of changes here:
- Add the GET /verify endpoint
- Email users a secret using MailGun
- Create a PendingAccounts table and record type
- Prefer do-notation for FromRow instances (and in general) instead of the <*>
  or a liftA2 style. Using instances using `<*>` makes the instances depend on
  the order in which the record's fields were defined. When combined with a
  "SELECT *", which returns the columns in whichever order the schema defines
  them (or depending on the DB implementation), produces runtime parse errors
  at best and silent errors at worst.
- Delete bill from accounts.csv to free up the wpcarro@gmail.com when testing
  the /verify route.

src/Accounts.hs

@@ -1,12 +1,26 @@
+{-# LANGUAGE RecordWildCards #-}
 {-# LANGUAGE OverloadedStrings #-}
 --------------------------------------------------------------------------------
 module Accounts where
 --------------------------------------------------------------------------------
 import Database.SQLite.Simple
 
+import qualified PendingAccounts
 import qualified Types as T
 --------------------------------------------------------------------------------
 
+-- | Delete the account in PendingAccounts and create on in Accounts.
+transferFromPending :: FilePath -> T.PendingAccount -> IO ()
+transferFromPending dbFile T.PendingAccount{..} = withConnection dbFile $
+  \conn -> withTransaction conn $ do
+    PendingAccounts.delete dbFile pendingAccountUsername
+    execute conn "INSERT INTO Accounts (username,password,email,role) VALUES (?,?,?,?)"
+      ( pendingAccountUsername
+      , pendingAccountPassword
+      , pendingAccountEmail
+      , pendingAccountRole
+      )
+
 -- | Create a new account in the Accounts table.
 create :: FilePath -> T.Username -> T.ClearTextPassword -> T.Email -> T.Role -> IO ()
 create dbFile username password email role = withConnection dbFile $ \conn -> do