Ability to edit pull request author

It should be possible to edit/change the pull request author.

Use case: When a developer has signaled (e.g., via JIRA issue status) that a branch is ready for review but forgotten to create a pull request, the reviewer should be able to create a pull request and then change its author to the developer. Without the ability to edit, the reviewer has to pick from two or three bad options:

1. delay the review until the developer creates a pull request (may take a few days!)
2. create a pull request with reviewer as an author, which allows the review to proceed but screws up subsequent interaction in many ways
3. review the branch without creating a pull request, which means review comments are going to be attached to individual branch commits (among other problems).