Make SpecialistSectorTagForm support existing tags

Until now, SpecialistSectorTagForm has only been used when creating a 
new tag, and not when editing an existing tag.

When updating an existing tag, it is important we keep the artefact 
up-to-date so that:
- the correct sector title will be indexed in search (for facet expansion)
- the artefact state will be the same as the tag state

To start doing this, SpecialistSectorTagForm has been refactored to 
accept a Tag instance on initialisation, and has changed the way that
attributes are assigned so that it is now a separate method call on the 
form object.