Allow 'No one' as an option for 'Allowed to Merge' in protected branches

In Protected Branches, the 'Allowed to Push' option includes a value for 'No one' under Roles. However, for 'Allowed to Merge', Roles does not include 'No one'.

The customer's use-case for setting both 'Allowed to Merge' and 'Allowed to Push' to 'No one' is to effectively make a particular branch read-only. If you consider GitLab's own branching strategy, we could use this for stable branches that have no possibility of a patch release, ensuring the branches remain untouched.