Refactor Core things, including Ecto handling, ActivityPub & Transmogrifier modules

* Data doesn't need anymore to be converted to ActivityStream format to
be saved (this was taken from Pleroma and not at all a good idea here)
* Everything saved when creating an event is inserted into PostgreSQL in
a single transaction

lib/service/federator.ex

@@ -53,7 +53,7 @@ defmodule Mobilizon.Service.Federator do
     Logger.debug(inspect(params))
 
     case Transmogrifier.handle_incoming(params) do
-      {:ok, activity, _} ->
+      {:ok, activity, _data} ->
         {:ok, activity}
 
       %Activity{} ->