Allow line breaks in LegislativeLists

([Trello ticket](https://trello.com/c/JORofGhy/253-manuals-make-legislative-list-work-0-5-day-spike))

Editors want to have multiple paragraphs in a single LegislativeList entry. In order to allow this, we need to modify the LegislativeList syntax to have an end tag, and treat everything inside it similarly.

Add $EndLegislativeList tag.

TODO:
- [ ] Update Govspeak help in specialist-publisher to reflect new LegislativeList syntax
This is odd syntax — I'd expect there to be a blank line above this one.