[BE] Distinguish user vs instance level permissions for granular scopes

Currently, granular scope permission access can be determined as follows:

if scope.all_membership_namespaces
  'all_memberships'
elsif scope.namespace.is_a?(::Namespaces::UserNamespace)
  'personal_projects'
elsif scope.namespace
  'selected_memberships'
else
  'user' # or 'instance'
end

We should add a way to distinguish user vs instance level access.

One way could be to add an enum field to granular scopes to determine access:

1. personal_projects
2. all_memberships
3. selected_memberships
4. user
5. instance

This will replace the all_membership_namespaces boolean column.