- Added tests to cover. - Added slight extra spaces at content joins.
- Removed ZIP system for now, until the idea can be fleshed out. - Added testing to cover. - Upgraded used library. - Added custom handling for BookStack callouts. - Added HTML cleanup to better produce output for things like code blocks.
Needed to tweak the default library strikethrough extension so that it uses the same element as front-end. Added testing to cover. For #2470.