Reassign book covers to dated folders and serve directly

[thomaswilburn]

The current approach of dumping all the covers into a single folder, predictably, is starting to get out of hand. We should:

• [x] Write a temporary task that moves them into dated subfolders based on the shelved data
• [x] Make sure any new download task places them in the correct subfolder
• [x] Update the templates to use the year automatically.
• [x] Set the server up to proxy requests for local /src/assets/sync directly into the folder instead of copying all of them into /build during startup.

[thomaswilburn]

Closed as of 3b70fa0