Allow only certain users to make changes without a pull request

STASH-2910 added the option for an additional (per-branch) restriction to block "Changes without a pull request"

However, this is an option that applies for all users, rather than "allowing only a restricted set of users to push but allowing all to merge a pull request" (which was the original summary to STASH-2910)

It would be very useful to have these additional options be able to be bypassed by certain users/groups. Use case:

• Commits to the release branches have to be done through pull requests, but the ci server (which isn't an access key user, so this isn't the same as STASH-5009) can do release builds and update versions in pom.xml files.

Note that the users with this permission must be managed separately to the list of users with write access to the branch, otherwise the use case of 'developers do their own merges to the branch after an approved pull request' won't work.

It may be useful to also apply this to the other two options added in stash 3.10 ("rewriting history" and "branch deletion"), but the pull request restriction is more useful, at least to me.