Invariant

Add support for tests that check invariance in the output. That way we have to validate every change that alters output.

Currently includes testing the build of the demo site against the contents of tests/data/baseline

Other things that could be tested: equivalence of base and jinja-base for example.


This includes a hack in gallery_rss that always adds the dc elements because they were only added on tests (I assume because of leakage from some other test, no idea)