[BE] Add policy to allow premium subscribers to view Roles and Permissions page

In #468353 (closed), we are adding default roles to the Settings -> Roles and Permissions page, which will show above the custom roles in the table. Currently, only Ultimate subscribers can view this page because custom roles is an Ultimate tier feature.

Because we now show default roles, we want Premium subscribers to see this page too. They will only see the default roles, not custom roles:

Our policy has :admin_member_role and :read_member_role, but both depend on custom_roles_allowed to be true. Since the Premium tier does not have the custom role feature, we can't use those 2 rules. We need a new rule that allows owners of a Premium subscription group to view the page.

backend also needs to send a boolean to frontend so that we can disable the New role button, since Premium subscribers can't create custom roles.