Update cabal file for 1.4 dependencies

- Re-freeze dependencies
- Lift these constraints during executable install on Travis

Temporarily move cabal.config so the executable-only install is free to use the
latest versions possible. By also installing within a sandbox on Travis, we
solve the dependency issue the same way we do in local development.
Upgrade code base for Yesod 1.4

- Replace Yesod.Text.Markdown shim
- Remove deprecated sqlOnlySettings
- Use SqlBackend, not SqlPersistT
- Add YesodAuthPersist instance
- Remove deprecated giveUrlRenderer