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

test/mobilizon_web/resolvers/comment_resolver_test.exs

@@ -18,7 +18,7 @@ defmodule MobilizonWeb.Resolvers.CommentResolverTest do
           mutation {
               createComment(
                   text: "#{@comment.text}",
-                  actor_username: "#{actor.preferred_username}"
+                  actor_id: "#{actor.id}"
               ) {
                 text,
                 uuid