Fix admins being able to suspend their instance actor (#14567)

* Fix admin being able to suspend their own instance account

* Add text about the instance's own actor in admin view

* Change instance actor notice from flash message to template

* Do not list local instance actor in account moderation list

app/policies/account_policy.rb

@@ -14,7 +14,7 @@ class AccountPolicy < ApplicationPolicy
   end
 
   def suspend?
-    staff? && !record.user&.staff?
+    staff? && !record.user&.staff? && !record.instance_actor?
   end
 
   def destroy?
@@ -62,6 +62,6 @@ class AccountPolicy < ApplicationPolicy
   end
 
   def memorialize?
-    admin? && !record.user&.admin?
+    admin? && !record.user&.admin? && !record.instance_actor?
   end
 end