Add IMPORTANT_NOTE actions.
Remove Edition.important_note field.

Add methods on Edition to add a new Action of type important_note, which
checks that a subsequent important_note_resolved does not exist, and a method
to resolve an existing important_note.
Change the way important_notes are resolved.

Instead of modifying request_type of existing note, we add a new Action of type
IMPORTANT_NOTE_RESOLVED.
Merge pull request #240 from alphagov/important_notes

Audit trail for important notes