Add unplug to dynamically load plugs

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

lib/service/unplug_predicates/app_config_keyword_equals.ex

@@ -0,0 +1,20 @@
+defmodule Mobilizon.Service.UnplugPredicates.AppConfigKeywordEquals do
+  @moduledoc """
+  Given an application and a key, execute the plug if the configured value
+  matches the expected value.
+  Usage:
+  ```elixir
+  plug Unplug,
+    if: {Unplug.Predicates.AppConfigEquals, {:my_app, :some_config, :enabled}},
+    do: MyApp.Plug
+  ```
+  """
+
+  @behaviour Unplug.Predicate
+
+  @impl true
+  def call(_conn, {app, key, keyword_key, keyword_default_value, expected_value}) do
+    app |> Application.get_env(key) |> Keyword.get(keyword_key, keyword_default_value) ==
+      expected_value
+  end
+end