Consolidate cover resize logic

Moves resizing out of the MusicBrainz namespace to Records - which
improves division of responsibilities, and removes double resizing
during import.

lib/music_library/records/cover.ex

@@ -0,0 +1,6 @@
+defmodule MusicLibrary.Records.Cover do
+  def resize(cover_data) do
+    {:ok, thumb} = Vix.Vips.Operation.thumbnail_buffer(cover_data, 600)
+    Vix.Vips.Image.write_to_buffer(thumb, ".jpg")
+  end
+end