Comment 6 for bug 138500

I had a very productive chat with ~sinzui about this, and we got a plan:

* We check for two levels of appropriateness: users that have any bugs assigned to them at all and users that are related to the project (they are either driver, owner, or have a bug in the project assigned to them) or any of the above indirectly, via team membership.
* When a user submits a form assigning the bug to someone, we check for these two conditions. If the assignee violates one of them, we continue to a new page, where the user will be presented with a warning, and will be able to select one of two options - `Continue` or `Back`. Choosing `Continue` will submit the changes and take the user back to the bug page, choosing `Back` will take the user to the bug page with the form prefilled with the values previously entered (so that they can change only the assignee). It may be nice, if possible, to also force-expand the form using JS.