Allow changing the owner of a pull request

Sometimes it makes sense for an administrator to re-assign a pull request to some other user - perhaps someone went on vacation, or perhaps the task just went to someone else.

With the proper permission it should be possible for a regular user to take over a pull request themselves. In Phabricator, this is called "commandeer", and I've found it very useful.

Lastly. some users need to be taken by the hand. It would be very useful for me to be able to create a pull request for someone that can't be bothered to learn how to create a PR, just to allow them to respond to code review comments. In these cases, it would be more appropriate to assign the PR to that user once I've created it.