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.

data/accounts.csv

@@ -1,3 +1,2 @@
 mimi,$2b$12$LynoGCNbe2RA1WWSiBEMVudJKs5dxnssY16rYmUyiwlSBIhHBOLbu,miriamwright@google.com,user,
-bill,$2b$12$wzh1OyNsvrrGt4hI52Wkt.QDX0IdPKn5uuNSgO/9CWucxipt5wlMi,wpcarro@gmail.com,manager,
 wpcarro,$2b$12$3wbi4xfQmksLsu6GOKTbj.5WHywESATnXB4R8FJ55RSRLy6X9xA7u,wpcarro@google.com,admin,