Allow Editor notes to have tags

www.agileplannerapp.com/boards/173808/cards/5154

Editor's notes were being validated as govspeak
causing safe html checks to fail. We've made an
exception in the SafeHTMLValidator to validate
only govspeak fields:

https://github.com/alphagov/govuk_content_models/pull/236
Merge pull request #273 from alphagov/bump-content-models-for-safe-html-change

Allow Editor notes to have tags
Add migration to import taggings from panopticon.

Requires bump to govuk_content_models.