Add tags and specialist_sectors to schema

The specialist sectors field is not indexed, because it's going to be
merged into the tags field.  It does need to appear in the "_source"
JSON of the document, though, so that amend actions work.  (Amend works
by fetching the _source field for the document, updating the fields
listed in the amendments, and then indexing it again.)

The tags field is just a non-analyzed field, holding a list of values,
which will be used for filtering and faceting.
Allow filtering, faceting and returning tag field
Populate the tag field when indexing

For now, just populate it from organisation and specialist_sector, since
those are the only ones of interest for now.
Add unit test of creation of tags field
Test that the tags field gets amended correctly

When its source fields are modified, the tags field should be updated.
Even if an amendment only sends some of the source fields, the tags
field needs to be populated with all the source fields.
Merge pull request #246 from alphagov/add_tag_field

Add tag field