Allow Pull Requests to source from a tag or commit as well as a branch

This is request to implement BSERV-3141 also for Bitbucket Cloud.

Some branch maintenance requires the ability to pull from an annotated tag (such as in the case of a QA certified commit) to downstream branches (our branchmasters use stash for convenience). Please consider including a tab in the "source" combo box choice for tags as well as branches.