Use filter key when creating subscriptions

Tom was right, I was wrong. I forgot that Specialist Publisher would be
posting emails with the field name as the tag key, rather than just
‘filter’. This fixes that.
Raise error if no choice selected, but choices available

This should not be possible, due to validation on front end
Fix subscriptions if no options are available
Merge pull request #135 from alphagov/readd-filter-key

Few fixes to email susbcriptions