Migration to rename 'housing' tags to 'housing-local-services'

The existing tag 'housing' will be used as a topic so we need to rename the existing tags.
Merge pull request #224 from alphagov/rename_housing_tags_to_housing_local_services

Migration to rename 'housing' tags to 'housing-local-services'
Fix error when attempting to create without a slug

This was attempting to register the blank slug with url-arbiter, which
returned a validation error.  This avoids calling url-arbiter for these
requests because we know they'll be invalid.
Merge pull request #225 from alphagov/fix_blank_slug_error

Fix error when attempting to create without a slug