Option to disable top-level group rename
Description
For Debian Salsa we want to force a particular namespace for top-level groups. To create groups, we have a separate interface, which enforces our naming schema. However users still could rename the groups by themselves. This is done to avoid username clashes with externally managed Debian user database while still allowing group creating by the users.
For now we have a hack in our GitLab version that disallows group renames.
Proposal
Add an option similar to username_changing_enabled that disallows top-level group renames except for admins.