CTFTime OAuth redirect to `/registration`

[chen-robert]

Ideally this would redirect to /registration while passing the token through using history state, however preact-router does not support attaching state (in JS) to a history entry. We can still implement this by implementing a state store using context and invalidate on any route change, which is enough to implement this.

Originally posted by @ethanwu10 in https://github.com/redpwn/rCTF/pull/200