1. Introduce contentcollector.js backend tests
1. Fix issue with OL LI items not being properly numbered after import
1. Fix issue with nested OL LI items being improperly numbered on export
1. Fix issue with new lines not being introduced after lists in on import #3961
1. Sanitize HTML on the way in (import)
1. Fix ExportHTML CSS because it needs to support OL > LI > OL not OL > OL [The latter being the correct format]
1. Fix backend tests.
The semantics of all the substitutions are identical to what they were before.
I _did_ take the liberty of formatting the CSS to be a bit more readable
(at the cost of adding a little bit of whitespace).