add documentation to teams for database structure

after converting to ActiveRecord I miss being able to see all of the
properties of a model in one file.
enforce team names be unique per competition

team names need to be unique for judging and clarity, but we want to
enable (and encourage!) repeat teams in different competitions.
add an interface to create teams

the form is VERY simple at the moment, but it's an intermediate stage. I
think this should actually appear on the competitions page as a simple
one-line form. perhaps an exercise in AJAX?