Add tag field

This adds a tag field to the index.

The tag field will contain a list of entries of the form "organisation:hm-revenue-customs" or "sector:oil-and-gas/licensing".  It may be used for facetting

If we need them, we'll add "primary_tag" field in future to hold the primary tags of each type (where that is a valid concept).  We could also parse tags which have a heirarchy into a separate field if we need to return parent tag facets, but we don't need that right now.

Also, and specialist_sectors to schema; these are received from clients, but are not indexed, only stored; they're actually indexed as part of the tag field, but need to be stored so that the amend action can work.

Pivotal story: https://www.pivotaltracker.com/story/show/73457476