Add a speakers page in the admin section

This is based on #270 as it shares a lot of the styling changes, so that should be merged first.

Add an additional page to show just speakers (i.e. users who have submitted sessions).
The implementation started out loading data from the database using Simple.Data, and so was a repository. It morphed away from that though as I removed duplication between it and the user profile repository. From the interface point of view, I think it should be consistent with the others (i.e. either they are are  query objects or all repositories).

I might rename all of the repository interfaces to something more query-based (although I don't like that term as it has quite database-specific connotations).