[Enhancement] Optionally use the YouTube API for improved fast indexing (#282)

* Started adding youtube API for fast indexing

* Hooked youtube API into fast indexing

* Added youtube_api_key to settings

* Added youtube api key to settings UI

* Added tests

* Refactored the youtube api module

* More refactor

* Changed editing mode name from basic to standard

* [WIP] started on copy changes

* Updated copy

lib/pinchflat/fast_indexing/youtube_behaviour.ex

@@ -0,0 +1,11 @@
+defmodule Pinchflat.FastIndexing.YoutubeBehaviour do
+  @moduledoc """
+  This module defines the behaviour for clients that interface with YouTube
+  for the purpose of fast indexing.
+  """
+
+  alias Pinchflat.Sources.Source
+
+  @callback enabled?() :: boolean()
+  @callback get_recent_media_ids(%Source{}) :: {:ok, [String.t()]} | {:error, String.t()}
+end