Use a task queue for talking to git server

[brianmoose]

There are multiple cases where the main CIVET server has to talk to the git server (like GitHub). If internet is slow or GitHub is down then it causes severe slowness in the CIVET server showing pages. We mitigate this somewhat by having timeouts when talking to the git server but it would be better to have a task queue like Celery to handle this. It could also handle failures and retry again in a certain amount of time. This is important in that when CIVET gets a pull request event it needs to do additional API requests to the git server. If these timeout then the event is not processed and is dropped.