Overhaul app cards

This commit tweaks the layout of app cards shown in browse and search
views to make them more visually parse-able.

- Make the app name bigger and use demi-bold font. There just isn't
  another way to make it stand out enough.
- Move the description/caption so that it's right under the app name, so
  the two are visually close and therefore associated in the user's mind.
- Move the category label to the bottom of the card, under the rating and
  align the two in their own little group that is separated from the app
  name + description group with whitespace. This reinforces that they
  are separate metadata.
- Move the backend name out of the Install button and instead display it
  as small, de-emphasized text in the corner. It's important that we
  still show this information to help disambiguate apps that appear
  multiple times but from different backends, due to mismatching
  appstream IDs between the instances causing Discover to be unable to
  de-duplicate them. However we don't need to shove this information in
  the user's face by sticking it in the Install button. So now it is
  demoted to a corner.
- Don't show the install button in compact/mobile view. There just isn't
  enough space on the cards without the layout looking really visually
  busy.