Use UuidMixin for EventSession

[jace]

As discussed in hasgeek/coaster#123, EventSession should replace its native UUID column with UuidMixin. A couple caveats though:

1. EventSession's uuid column has nullable=True defined. This will need a migration.
2. Nearly half the data has null values for session UUID despite a default value on the column. It's unclear why and needs investigation.

[jace]

As far as I can tell, there's no bug (despite the claim in a3046b2). The uuid column was introduced after there were already 1.5 million sessions in production, and we simply didn't bother to generate UUIDs for them. All later sessions have UUIDs, and the code doesn't let that slip.