Add config endpoint

lib/mobilizon/common-config.ex

@@ -0,0 +1,7 @@
+defmodule Mobilizon.CommonConfig do
+  def registrations_open?(), do: instance_config() |> get_in([:registrations_open])
+
+  def instance_name(), do: instance_config() |> get_in([:name])
+
+  defp instance_config(), do: Application.get_env(:mobilizon, :instance)
+end