switch to immutable primary keys

[chadwhitacre]

For a couple tables we're using mutable strings for primary key (participants.username and teams.slug). This makes it easier to debug things because you don't have to dereference opaque numeric ids when querying other tables, but it's rather non-standard and might potentially lead to real trouble. Should we switch over to bigserials like the rest of the world?

[chadwhitacre]

Closing as a dupe of #835.