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

priv/repo/migrations/20191023143659_move_participants_stats_to_event.exs

@@ -0,0 +1,15 @@
+defmodule Mobilizon.Storage.Repo.Migrations.MoveParticipantsStatsToEvent do
+  use Ecto.Migration
+
+  def up do
+    alter table(:events) do
+      add(:participant_stats, :map)
+    end
+  end
+
+  def down do
+    alter table(:events) do
+      remove(:participant_stats)
+    end
+  end
+end