Add get/set to TabSettings model

This allows us to treat the TabSettings object like a dictionary,
getting and setting values to it backed by a database. This does not
change the fields of the table, so no migration is needed. Note that in
general using the cache is preferred to using this new interface, but if
you need the data persisted this is an easy way to do it.
Added the ability to release Pairings

There are now two actions "Release pairings" and "Assign Judges" that
tab staff can do from the pairing page. The former will make it so that
people going to the mit tab landing page can get to the pairings or not.
Touching up the home page, and landing page.

We should probably not be showing navigation on login, so I removed
navigation on login. I also edited some of the copy to make it more
accurate.