Change Channel to Source (#12)

* Ensure channel detail lookup doesn't download the video - oops

* Ran the needful migrations for replacing channels with sources

* got media source test back working

* channel tasks test

* Media indexing worker test

* media tests

* Got all tests working except controller tests

* got all tests working

* Renamed Channel struct to Source

* renamed ChannelTasks

* More renaming; renamed channel details module

* Removed Channel yt-dlp module, instead throwing things in the VideoCollection module

* Renamed what looks like the last of the outstanding data

test/support/fixtures/media_fixtures.ex

@@ -16,7 +16,7 @@ defmodule Pinchflat.MediaFixtures do
         media_id: Faker.String.base64(12),
         title: Faker.Commerce.product_name(),
         media_filepath: "/video/#{Faker.File.file_name(:video)}",
-        channel_id: MediaSourceFixtures.channel_fixture().id
+        source_id: MediaSourceFixtures.source_fixture().id
       })
       |> Pinchflat.Media.create_media_item()