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

.formatter.exs

@@ -3,5 +3,5 @@
   subdirectories: ["priv/*/migrations"],
   plugins: [Phoenix.LiveView.HTMLFormatter],
   inputs: ["*.{heex,ex,exs}", "{config,lib,test}/**/*.{heex,ex,exs}", "priv/*/seeds.exs"],
-  line_length: 100
+  line_length: 120
 ]