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/plugs/uploaded_media_plug_test.exs

@@ -18,8 +18,7 @@ defmodule MobilizonWeb.Plugs.UploadedMediaPlugTest do
     }
 
     {:ok, data} = Upload.store(file)
-    [%{"href" => attachment_url} | _] = data["url"]
-    [attachment_url: attachment_url]
+    [attachment_url: data.url]
   end
 
   setup_all :upload_file