Remove jQuery from `ghost_footer` helper.

closes #1161
- Remove the script that adds jQuery to the `ghost_foot` helper.
- Add comment so future devs know why `ghost_foot` doesn’t currently do anything.
- Update test to expect the result of `ghost_foot` to be an empty string.
- Update Casper theme to include jQuery in a more direct manner (see TryGhost/Casper#68).