Data migration to update LegislativeList syntax

This will be introduced with the next version of govspeak, requiring a closing tag.

Opened for comment as after this is deployed, but before the migration has run, LegislativeLists in attachments will not display correctly.

- [ ] Merge https://github.com/alphagov/govspeak/pull/38
- [ ] Release new govspeak version and update in whitehall gemfile
- [ ] Alter LegislativeList explanation in help sidebar