Allow fallback to front or back image for thumbnail if icon image is not set

[TheLastProject]

Fixes #1929

• Write code to try icon first, then fall back to front or back
• Generate header colour correctly (right now it always is based on the icon or random)
• Fix the notable slowdown when scrolling through the main activity

To decide: how should this interact with #1889?

[TheLastProject]

The slowdown seems to be caused by BitmapFactory.decodeStream(in); in Utils.retrieveCardImage.

This is generally just bad code, it should try to load the image async. It's not really directly linked to this, but if we're going to silently start loading more stuff we should consider being a bit more careful.

[TheLastProject]

Closing in favour of #1934 and #1933