Remove assumption that the arena is called 'main'

[PeterJCLaw]

The name of the arena is a variable within SRComp. This is primarily to cope with supporting more than one arena, though also to allow event organisers to choose whatever names they like (for signage etc.)

The livestream overlay should not assume the name of the arena is a constant.

The details of arenas can be gathered from the arenas enpoints. For the single arena case, the overlay shouldn't care what the name of the arena is.

The overlay should probably cope with multiple arenas, though that's probably a separate issue.