Generalise zone support

[PeterJCLaw]

There are two parts to this:

• generalise zone colours to come from the API
• generalise the display of teams in zones, so that the number of zones is determined by the match data

Together these move the responsibility for the arena definition into the API, rather than hardcoding it into the overlay.

Fixes https://github.com/srobo/competition-team-minutes/issues/256