Fancy directory listing for created images

Adjust the templates and related code to make the directory listings of images.postmarketos.org/bpo and subdirs look good, and extend it with useful information, so we can simply link to that page from the main postmarketOS homepage. It should be obvious to people which images they need to download for their use case.

TODO:

• add some CSS to make it not ugly (- )maybe make the whole thing somewhat look like the main homepage?)
• link back to main postmarketOS homepage
• subdir with devices: list full device names at the bottom, so people can find them more easily (android code names are usually completely different from their proper names)
• subdir with image files: add device specific install intructions
• create MR
• add "downloads" page to pmOS homepage, link to images.postmarketos.org/bpo -> postmarketos.org!99 (merged)