Allow sector tags to be stored for an Artefact

Add 'sectors' to the list of tags which can be added to an Artefact.
A sector tag is a section-like term to group GOV.UK content (largely
detailed guides) by the industry sector which it applies to. Like 
sections, sectors can be nested in a hierarchy.
Change 'sector' to 'industry_sector'
Merge pull request #108 from alphagov/add-sector-tag-type

Allow sector tags to be stored for an Artefact