Ratelimit checking (for github)

[mcbloch]

Add ratelimit checking for github.

Maybe save this value locally to show the remaining requests in the frontend.
Disable sync requests when the rate is exceeded.

Optional: Add ratelimits for clients themselves.
Optional: Limit the amount of syncs per type when no oauth token is provided because you only get 60 unauthorized api requests per hour.

https://developer.github.com/v3/#rate-limiting