Talk attending status

Can't remember whether this was required or not over talk favouriting, but here goes...

This adds the ability to mark a talk as attended or not.  It's pretty much based on the event attending code, so anything that happens there should happen here.

* GET request to `/talk/<ID>/attending` when not authenticated, returns `false`
* GET request to `/talk/<ID>/attending` when authenticated, returns the attending status for the authenticated user
* POST request to `/talk/<ID>/attending` when authenticated, marks the user as attending
* DELETE request to `/talk/<ID>/attending` when authenticated, marks the user as not attending

The talk output also now includes an `attending` field and an `attendee_count` field, with relevant data in it.

Frisby tests updated to include the new fields for talks.