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)

these actions will have `created_at` and `updated_at` same as the time the migration was run. i understand that we've not tracked the exact time the important note was added, and at best can make a close guess: which is more likely to be the `updated_at` for the `edition`, than `Time.now`. what do you think?

also, in data migrations i prefer using `create!` so that validation errors raise an exception, and we're stopped from creating invalid data. it might not be a top concern with `Action` but nice to have.