Build device images with bpo

In order to provide up to date images on the homepage for various UIs and devices (https://gitlab.com/postmarketOS/postmarketos.org/-/issues/131), they shall be queued and built with builds.sr.ht jobs managed by build.postmarketos.org (bpo).

• image jobs should not run in parallel with package build jobs, so we don't overload builds.sr.ht
• each image job should build one UI for one device - not more, so it can easily be restarted and does not block our build slot for too long
• new image jobs should get added automatically (make it configurable, let's start with "weekly")
• old images should get removed after some time so we don't use up too much disk space (only delete images generated by bpo)

Related: https://gitlab.com/postmarketOS/postmarketos-images/

I'll work on this in November 2020 March 2021 December/January 2020/2021.

@milesalan: FYI, we can also provide Sxmo images with this once it's implemented.