Introduce group basic federation, event new page and notifications

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

priv/repo/migrations/20200406115807_add_conversations.exs

@@ -0,0 +1,18 @@
+defmodule Mobilizon.Storage.Repo.Migrations.AddConversations do
+  use Ecto.Migration
+
+  def change do
+    create table(:conversations) do
+      add(:title, :string)
+      add(:slug, :string)
+      add(:actor_id, references(:actors, on_delete: :delete_all), null: false)
+      add(:creator_id, references(:actors, on_delete: :nilify_all))
+      add(:last_comment_id, references(:comments, on_delete: :delete_all))
+      timestamps()
+    end
+
+    alter table(:comments) do
+      add(:conversation_id, references(:conversations, on_delete: :delete_all))
+    end
+  end
+end