Improve layout
Update to use new http API

Means only one HTTP request occurs rather than two.
Emit an error if the corner or arena isn't valid.

This covers both the cases where it isn't present, and when the
corner doesn't exist. The lack of existence of the arena will appear
shortly.
Use search params for arena/corner specification.

This is easier to read than an anchor, and features the major
advantage that it will refresh the page when you change it.
Add links to suitable corners on invalid startup.

The links are generated by querying the backend for suitable values.
This makes it much easier to both develop and the screens, since
there are nice links to each of the available corners.