Admin -> Challenges: Support manual challenge resync

[sei-bstein]

When meaningful events happen around challenges (e.g. grading, regrading, starting, stopping, etc.) Gameboard synchronizes its representation of the challenge's state with the game engine that is running it (e.g. Topomojo). We don't currently expose a way to manually initiate that process.

There's no situation under normal usage that would require a user to do this, but we've found ourselves in situations where it'd have been nice to be able to, usually in test or QA environments. Providing this capability would addresse that need while also potentially acting as a last-ish-resort support option for others.

[sei-bstein]

Drop on manage challenges modal