Added OPML Endpoint for podcast rss feeds (#512)

* Added OPML Endpoint for podcast rss feeds

* changed opml route and added controller test for opml endpoint

* add copy opml feed button

* add copy opml feed button - correct url

* fix html indenting

* add indentation to opml

Co-authored-by: Kieran <kieran.eglin@gmail.com>

* use convention for unused controller params

Co-authored-by: Kieran <kieran.eglin@gmail.com>

* add test for opml_sources helper function

* change opml endpoint to be more inline with the other routes

---------

Co-authored-by: robs <git@robs.social>
Co-authored-by: Kieran <kieran.eglin@gmail.com>

test/pinchflat/podcasts/podcast_helpers_test.exs

@@ -6,6 +6,16 @@ defmodule Pinchflat.Podcasts.PodcastHelpersTest do
 
   alias Pinchflat.Podcasts.PodcastHelpers
 
+  describe "opml_sources" do
+    test "returns sources not marked for deletion" do
+      source = source_fixture()
+      source_fixture(%{marked_for_deletion_at: DateTime.utc_now()})
+      assert [found_source] = PodcastHelpers.opml_sources()
+      assert found_source.custom_name == source.custom_name
+      assert found_source.uuid == source.uuid
+    end
+  end
+
   describe "persisted_media_items_for/2" do
     test "returns media items with files that exist on-disk" do
       source = source_fixture()