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/mobilizon_web/resolvers/picture.ex

@@ -51,7 +51,7 @@ defmodule MobilizonWeb.Resolvers.Picture do
         %{context: %{current_user: user}}
       ) do
     with {:is_owned, %Actor{}} <- User.owns_actor(user, actor_id),
-         {:ok, %{"url" => [%{"href" => url, "mediaType" => content_type}], "size" => size}} <-
+         {:ok, %{name: _name, url: url, content_type: content_type, size: size}} <-
            MobilizonWeb.Upload.store(file),
          args <-
            args