Cleanups 1

Cleaned up the usage of choices a bit. Django adds a .get_X_display() function to the model for every field with choices.

Also added nullable foreign keys in the ladder module. Once these are live on the site they will be enabled on the Eros server.

Added player profile links to the match history.

I haven't tested any of these changes yet. Someone with a dev environment set up may wish to confirm that they all work before merging.