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/person.ex

@@ -155,7 +155,7 @@ defmodule MobilizonWeb.Resolvers.Person do
       if Map.has_key?(args, key) && !is_nil(args[key][:picture]) do
         pic = args[key][:picture]
 
-        with {:ok, %{"name" => name, "url" => [%{"href" => url, "mediaType" => content_type}]}} <-
+        with {:ok, %{name: name, url: url, content_type: content_type, size: _size}} <-
                MobilizonWeb.Upload.store(pic.file, type: key, description: pic.alt) do
           Map.put(args, key, %{"name" => name, "url" => url, "mediaType" => content_type})
         end