Only check if user has moderator role in is_moderator? method

In the beginning of the content_moderation feature we considered
user that have the admin or stuff roles to be moderator. But now
we have the dedicated moderator role. So we only should check
for that in the `is_moderator?` method and leave the rest to
the pundit policies.

src/api/app/models/user.rb

@@ -409,7 +409,7 @@ def is_nobody?
   end
 
   def is_moderator?
-    is_admin? || is_staff?
+    roles.exists?(title: 'Moderator')
   end
 
   def is_active?

src/api/app/policies/decision_policy.rb

@@ -2,6 +2,6 @@ class DecisionPolicy < ApplicationPolicy
   def create?
     return false unless Flipper.enabled?(:content_moderation, user)
 
-    user.is_admin? || user.is_moderator?
+    user.is_moderator? || user.is_admin? || user.is_staff?
   end
 end