Move publish tag button to new right-hand column

This splits the form into a layout with better-defined columns, and
moves the “Publish” button to the right-hand column.

We decided to move the button so that it appears conceptually separate
from the main form so that it doesn’t imply "Save and publish" behaviour.
Keep the state of the artefact up-to-date with the tag

This changes the SpecialistSectorTagForm and
UpdateSpecialistSectorTagArtefactObserver so that we keep the artefact
associated with the tag in sync with its state.