Improve event creation form by introducting EventOptions

It's a subentity that holds additional metadata in a map database type

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

priv/repo/migrations/20190902082656_add_options_to_event.exs

@@ -0,0 +1,9 @@
+defmodule Mobilizon.Repo.Migrations.AddOptionsToEvent do
+  use Ecto.Migration
+
+  def change do
+    alter table(:events) do
+      add(:options, :map)
+    end
+  end
+end