Support Debian stable as a source for ldh_synapse
User story: I am a Liberty developer targeting Debian stable. I'd like to install Synapse using a Debian-managed source, so I can benefit from Debian integration and maintenance.
Suggested solution: Extend Shipwright to support stretch-backports as the default source for installing Synapse. (Keep Matrix-upstream as an alternate source.) Blocked by https://bugs.debian.org/919707 - see #12 (closed) for details.