[WIP] Create a new Review Request after creating a PR on Github

Review Request #8437 — Created Sept. 25, 2016 and updated — Latest diff uploaded

Review Board

This is mainly exploratory code.

Add support for automatically creating a review request
from a pull request.

Step one in integrating pull requests with Reviewboard is
to first create the review request when a PR happens.
For now, we provide support only for Github.

To set up the webhook, first make sure your repository
is linked to Reviewboard. Then, in your repo's settings on Github,
select 'Webhooks' and add
Make sure to unselect 'Push' and select 'Pull Request' for the event
you'd like to know about.

A new review request should now be created, and the ID of the pull request
that the review request was created from should be displayed on the
"Information" panel.

Note that if you're testing locally, you should
set up localtunnel
to foward your request.

Now when you make a commit, a review request should
be created automatically.

Still to do
Sync more data like branch info
Import diffs from the PR
* Link user from Github to Reviewboard (and handle cases where it doesn't exist)