Host webrev on Betterrev site

[Martijn Verburg]

Once we've created the webrev, we'll need to host it on the user's behalf.

[arkangelofkaos]

Questions to think about: What level of security will we need on this hosting? When will the webrev be deleted?

[Richard Warburton]

I don't think we need any security on web revs. It's just a pull request on an open source project.

I think we can delete the webrev after it has been merged or terminated. There are events in the state transition diagram that represent these events.

[arkangelofkaos]

Currently the method on the PullReview object to get the webrev returns a file. This needs to be changed once we set up hosting for the webrev so that the method returns a URL.