Ability to update pull request description with new commits

When creating a Pull Request, BB sets a description based on the commit messages in the PR.

When auto-updating, this is not done (understandably).

Solutions:

1. Auto-update the description (not desirable for obvious reasons)
2. Include a Button when Updating a PR to automatically populate the description from the PR
3. A hybrid approach (using a "macro" embedded in the description or a flag that says that the description should track the commits)