Build whole postmarketOS images

Add a new config in bpo/config/const/images.py to define which images shall be built and in which interval. Add a new "image" table, similar to "package". On startup, fill the image queue based on the config, and remove obsolete database entries. Create branch:device:ui specific jobs once all packages for a given branch are built, and a slot for starting another build job is available. Packages are always higher prioritized than images. Run the queue full function periodically, so we can add new entries once the interval is reached.

Finished images are uploaded into one directory per database entry. That directory gets a readme.html based on a new template, and it links to the log entry that built the files in the directory.

Along with the big commit that introduces building whole postmarketOS images come several commits that refactor the code to make this possible. Refer to the individual commit messages for details.

Close: #76 (closed)