Install executable-only packages outside of the sandbox

The dependencies of our executable-only packages and our application seem to
conflict. We're unable to determine the root cause, but installing the
executable-only packages outside of the sandbox solves the issue.

This also means these packages can be reused across projects. If we assume there
aren't as strict version requirements, this should be a Good Thing.
Update cabal file for 1.4 dependencies
Re-freeze dependencies
Replace Yesod.Text.Markdown shim
Remove deprecated sqlOnlySettings
Use SqlBackend, not SqlPersistT
Add YesodAuthPersist instance
Remove deprecated giveUrlRenderer
Lift version constraints on 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.