Refactor Mobilizon.Federation.ActivityPub and add typespecs

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

test/graphql/resolvers/group_test.exs

@@ -57,7 +57,7 @@ defmodule Mobilizon.Web.Resolvers.GroupTest do
         |> post("/api", AbsintheHelpers.mutation_skeleton(mutation))
 
       assert hd(json_response(res, 200)["errors"])["message"] ==
-               "A group with this name already exists"
+               "A profile or group with that name already exists"
     end
   end
 

test/graphql/resolvers/resource_test.exs

@@ -598,7 +598,7 @@ defmodule Mobilizon.GraphQL.Resolvers.ResourceTest do
           }
         )
 
-      assert is_nil(res["errors"])
+      assert res["errors"] == nil
 
       assert res["data"]["updateResource"]["path"] ==
                "#{root_folder.path}/#{@updated_resource_title}"