Allow industry sector tags to be assigned to artefacts

This adds a new select element to the artefact form, so that industry 
sector tags can be assigned to artefacts.
Extract tag helper methods into separate helper
Include top-level tags in tag lists

There are use cases where things may need to be tagged with a top-level 
tag in a tag type. To make this play nice within a multi-select field 
and optgroups, this inserts the parent tag at the start of the optgroup.
Merge pull request #115 from alphagov/industry-sectors-tag-ui

Allow industry sector tags to be assigned to artefacts