Work on actors

* Implement group GraphQL APIs
* Change Actors changeset to properly set urls
* Remove old actors indexes and add some new ones

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

test/mobilizon_web/resolvers/person_resolver_test.exs

@@ -8,7 +8,7 @@ defmodule MobilizonWeb.Resolvers.PersonResolverTest do
   @non_existent_username "nonexistent"
 
   describe "Person Resolver" do
-    test "find_actor/3 returns a person by it's username", context do
+    test "find_person/3 returns a person by it's username", context do
       {:ok, %User{default_actor: %Actor{} = actor} = _user} = Actors.register(@valid_actor_params)
 
       query = """