Layout Sketches for the App Details Design

This issue is to host discussion about designs being worked on by @crwinfrey as part of the Bazaar2 funding. The designs here are specific to the App Details screen.

Some things that were briefly touched on in our first meeting about this include:

• How can donations be made more prominent (#232 (closed))
• Making versions less prominent (but still accessible)
• Tapping installed version should not remove the installed app (#316 (closed))
• Adding "Swap with F-Droid" to the sharing menu
• Adding feature image artwork if available (or a stock design if not)

There are other issues throughout the tracker which touch on aspects of this design, and they will be linked with this issue over time.

This is a sibling of #709 (closed) and #742 (closed) which show a series of sketches about the main app design (list of apps, searching, updating, notifications, etc).