Allow users to edit the destination of a pull request (BB-3651)

Oftentimes, pull request changes are made directly to the 'default' branch of a repo. That's generally where the main developers do their stuff. This often means that external pull request changes will conflict, which makes integration with the pull request API impossible. You have to merge things locally and stuff.

It would be better to pull the changesets over and immediately fork them into a new branch. This process would ensure that the 'default' branch would remain working and reasonable, and that the developers could merge the new branch into the main line as needed. Such merging could involve the creation of additional changesets, like adding appropriate tests to a bug fix, and so forth. All of this info would be tracked, and everything would make sense.