Add name/project uniqueness constraint to ProtectedBranches

We do not enforce any kind of uniqueness requirement on the name of new protected branches. It seems like its something we'd want to add, as we often do name searching (via RefMatcher#matching etc) This change might be as "simple" as add_index :protected_branches, [:project_id, :name], unique: true

(Spotted in !15862 (merged))