refactor(backend): change naming of function names to avoid the is_ prefix

Following Credo.Check.Readability.PredicateFunctionNames check

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

test/tasks/actors/new_test.exs

@@ -179,7 +179,7 @@ defmodule Mix.Tasks.Mobilizon.Actors.NewTest do
       assert %Actor{name: @group_name, preferred_username: @group_username, id: group_id} =
                Actors.get_group_by_title(@group_username)
 
-      assert Actors.is_administrator?(admin_id, group_id)
+      assert Actors.administrator?(admin_id, group_id)
     end
   end
 end