Prevent loading setup screen if already setup

Closes #3145
- Prevent navigation to the setup screen if Ghost setup
  has previously been completed.
- Fix templates that were incorrectly using foreach instead of each.
- Add validation for minimum password length.
- Fix up functional tests and split out tests for setup to a separate
  instance of casper because setup requires a new database.
- Add a cleanDatabase task to grunt which resets the database to
  new.