Just a few small tweaks

• Description expansion animates now. Less jarring than everything jumping down
• The header appbar will now stop being transparent when the user scrolls down
  • Might be worth revisiting this and migrating it to Paper's Appbar component?
• The pull down refresh spinny thing now has the proper colors
• Fixed download status not updating when using the "Delete all chapters" option in the header menu. Looks like a bad copy-paste from when that feature was added