Move SpecialistSector.options_for_select to helper

As massaging the data into a format that Rails helpers will accept is a
view concern we've split this part out.

Adds SpecialistSector.grouped_sector_topics which fetches specialist
sector tags from panopticon and turns them into a more useful data
structure.

Adds EditionsHelper.specialist_sector_options_for_select which takes
those grouped topics and turns them into the nested array format that
Rails expects.
Only show specialist sectors field when content API available

If the content API is unavailable the field isn't rendered which allows
editors to continue with their work as normal. Any specialist sector
associations on the drafts being saved will remain unchanged.
Fix typo in specialist sector scenario