Audit trail for important notes

Add author/timestamp to important notes, and show them in the edition history.

Depends on https://github.com/alphagov/govuk_content_models/pull/240 (and subsequent version bump) which removes the important_notes field on Edition and makes important notes another type of Action. This PR includes a migration to convert the existing notes into Actions.

Slightly changes the behaviour, in that it is no longer possible to update an existing note; now you can either add a new one (overriding the previous one, which is still shown in the history) or click a separate button to resolve the existing one (which actually adds another Action marking the note as resolved).

![image](https://cloud.githubusercontent.com/assets/131395/4921536/f37aa302-650a-11e4-9637-321b8120bc7a.png)