Redo indexing mechanism (#16)

* Bumped up the line length because I fear no man

* Refactored indexing

Previously, indexing worked by collecting the video IDs of only videos
that matched indexing criteria. This new model instead stores ALL videos
for a given source, but will only _download_ videos that meet that criteria.
This lets us backfill without indexing, makes it easier to add in other
backends, lets us download one-off videos for a source that don't quite
meet criteria, you name it.

* Updated media finders to respect format filters; Added credo file

test/support/fixtures/media_fixtures.ex

@@ -15,6 +15,8 @@ defmodule Pinchflat.MediaFixtures do
       |> Enum.into(%{
         media_id: Faker.String.base64(12),
         title: Faker.Commerce.product_name(),
+        original_url: "https://www.youtube.com/channel/#{Faker.String.base64(12)}",
+        livestream: false,
         media_filepath: "/video/#{Faker.File.file_name(:video)}",
         source_id: MediaSourceFixtures.source_fixture().id
       })