Ability to change the author of a pull request

Copy of BSERV-8635

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.

Other use cases from BSERV-8635:
Another reasonable use case is when the developer leaves the team/company. Typically projects get reassigned for maintenance/continuation purposes and it would be great if PR owners could be updated respectively.

There are times when a second developer has taken over the work that the first has done, and in some circumstances changed everything in it... at this point the second developer should be able to become the author of that pull request

Without the ability to edit, the reviewer has to pick from two or three bad options:

delay the review until the developer creates a pull request (may take a few days!)
create a pull request with reviewer as an author, which allows the review to proceed causes subsequent interaction and understanding
review the branch without creating a pull request, which means review comments are going to be attached to individual branch commits (among other problems).