'Remove source branch' is displayed to mergers who cannot actually delete the source branch from a forked project

On MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11447 by @deckar01, created on his own fork, I see the "Remove source branch" checkbox even though I don't have push permissions to his fork, which means that that deletion will not actually go through.

Since the "Delete source branch when the merge request is accepted." checkbox was removed from the "New MR" form with the introduction of the new MR merge widget, there is now also no longer a way for @deckar01 himself to indicate that the source branch should be deleted when the MR is accepted. This is now only something that the MR merger can do, who (as said) cannot actually perform that deletion!

I think we should restore the "Delete source branch when the merge request is accepted." checkbox in the form, and check that by default, and restore the behavior we had in the past, where the MR widget has a "Remove source branch when merged" button that could be clicked by everyone with branch-delete access to the source project, which doesn't necessarily include the MR merger, and a "Delete source branch" checkbox that is only displayed when the merger actually has branch-delete access.

/cc @fatihacet @victorwu