Consider regression testing against the last N versions of Synapse, plus develop

[turt2live]

Ideally as other server implementations become popular we just add to our regression tests, but for now Synapse holds the market share.

With an initial suggestion of N being 3 minor versions (1.x series, not patch versions) plus one for develop we could catch bugs in upcoming releases of Synapse and ensure our upcoming release is backwards compatible too.

This should be automated and happen concurrently with a deploy to /develop (so we don't block features behind an hour of regression tests). We could also run them nightly with the electron stuff, but again we shouldn't block the electron nightlies from publishing because of this.