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/events/event_participant_stats.ex

@@ -0,0 +1,44 @@
+defmodule Mobilizon.Events.EventParticipantStats do
+  @moduledoc """
+  Participation stats on event
+  """
+
+  use Ecto.Schema
+  import Ecto.Changeset
+
+  @type t :: %__MODULE__{
+          not_approved: integer(),
+          rejected: integer(),
+          participant: integer(),
+          moderator: integer(),
+          administrator: integer(),
+          creator: integer()
+        }
+
+  @attrs [
+    :not_approved,
+    :rejected,
+    :participant,
+    :moderator,
+    :administrator,
+    :moderator,
+    :creator
+  ]
+
+  @primary_key false
+  @derive Jason.Encoder
+  embedded_schema do
+    field(:not_approved, :integer, default: 0)
+    field(:rejected, :integer, default: 0)
+    field(:participant, :integer, default: 0)
+    field(:moderator, :integer, default: 0)
+    field(:administrator, :integer, default: 0)
+    field(:creator, :integer, default: 0)
+  end
+
+  @doc false
+  @spec changeset(t, map) :: Ecto.Changeset.t()
+  def changeset(%__MODULE__{} = event_options, attrs) do
+    cast(event_options, attrs, @attrs)
+  end
+end