Backport 'Move frontend virtualhost logic on backend'

This essentially aims to backport

nexedi/slapos!1435 and
nexedi/slapos!1504

to old slapos tag 1.0.249.

Some patches are deliberately skipped:

1. nexedi/slapos@5b3fc1f2

We don't need this Zope backport fix, because this fix
aims to fix a patch that isn't even present in the Zope
we are using. See here for more context:

https://github.com/zopefoundation/Zope/issues/1191

2. nexedi/slapos@d49914a6

This seems to be a complex change that we may not need for
our purposes.