move to official Debian base box

This should move to using the official Debian boxes for the base box, instead of having our own custom basebox.

• move as many package deps as possible to the build metadata fdroidserver!925 (closed)
• move any required basebox customizations to buildserver/provision-apt-get-install (fdroidserver#907 (closed))
• figure out how to extend the filesystem on Debian boxes, they default to 20GB root filesystem.
• archive https://gitlab.com/fdroid/basebox project

The standard provisioning script can then also be reused to generate Docker images for CI, etc. This would then let us merge the buildserver, ci-images-client, and the fdroiddata fdroid build job setup into the same project.

./makebuildserver now ends up producing a box that is ~18GB, so it might be possible to resize after that stage.

@proletarius101 @uniqx FYI