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/mix/tasks/mobilizon/move_participant_stats.ex

@@ -0,0 +1,67 @@
+defmodule Mix.Tasks.Mobilizon.MoveParticipantStats do
+  @moduledoc """
+  Temporary task to move participant stats in the events table
+
+  This task will be removed in version 1.0.0-beta.3
+  """
+
+  use Mix.Task
+
+  alias Mobilizon.Storage.Repo
+  alias Mobilizon.Events
+  alias Mobilizon.Events.Event
+  alias Mobilizon.Events.ParticipantRole
+  import Ecto.Query
+
+  require Logger
+
+  @shortdoc "Move participant stats to events table"
+  def run([]) do
+    Mix.Task.run("app.start")
+
+    events =
+      Event
+      |> preload([e], :tags)
+      |> Repo.all()
+
+    nb_events = length(events)
+
+    IO.puts(
+      "\nStarting inserting participants stats into #{nb_events} events, this can take a while…\n"
+    )
+
+    insert_participants_stats_into_events(events, nb_events)
+  end
+
+  defp insert_participants_stats_into_events([%Event{url: url} = event | events], nb_events) do
+    with roles <- ParticipantRole.__enum_map__(),
+         counts <-
+           Enum.reduce(roles, %{}, fn role, acc ->
+             Map.put(acc, role, count_participants(event, role))
+           end),
+         {:ok, _} <-
+           Events.update_event(event, %{
+             participant_stats: counts
+           }) do
+      Logger.debug("Added participants stats to event #{url}")
+    else
+      {:error, res} ->
+        Logger.error("Error while adding participants stats to event #{url} : #{inspect(res)}")
+    end
+
+    ProgressBar.render(nb_events - length(events), nb_events)
+
+    insert_participants_stats_into_events(events, nb_events)
+  end
+
+  defp insert_participants_stats_into_events([], nb_events) do
+    IO.puts("\nFinished inserting participant stats for #{nb_events} events!\n")
+  end
+
+  defp count_participants(%Event{id: event_id}, role) when is_atom(role) do
+    event_id
+    |> Events.count_participants_query()
+    |> Events.filter_role(role)
+    |> Repo.aggregate(:count, :id)
+  end
+end