Started work on moderators system.

There's a new migration, ModsForGame_users_to_moderator (the filename is
different, but that's what I passed into `rails g migraiton`. We made
some changes to the ModsForGame model, but not the migration to create
the database, which caused some problems. That's now fixed.

Additionally, if you look at either the new/edit (anything that uses the
ModsForGame form partial) it uses dropdowns for selecting a user and
game, so that we don't have to remember ids or some other identifier. It
also made doing things easy (no controller changes).

As for the ModsForGame model, you can now call ModsForGame.moderator,
and it'll return a User object. Huzzah! No changes were needed for the
Game object, because we got that one right the first time.

There might need to be similar changes for other join tables. Just a
heads up (if anyone is reading this).

Additionally, just so that things are easy to read, the User and Game
models now have a to_s method. Users display `[name] <[email]>`,
games display `Game <[start_date] - [end_date]>`. It looks nicer. The
game one I'm not beholden to, but I like the user one. If you just want
their name, use user.name...

Additionally, I got rid of the link (but not references, you can still
access it by url) to the show mods_for_game, because it doesn't tell you
anything. There's now just edit and destroy.