In gitlab-org/gitlab-ce!13251 wildcard Protected Branches were handled
properly when deleting all merged branches. But this fix wasn't that
good. It also checked branch names against Protected Tags. That's not
correct.
This change will **only** check if there is a Protected Branch
matching the merged branch, and ignores Protected Tags.
Closesgitlab-org/gitlab-ce#39732.
It adds a button to the branches page that the user can use to delete
all the branches that are already merged. This can be used to clean up
all the branches that were forgotten to delete while merging MRs.
Fixes#21076.