The behaviour of #TERM in search has been amended
1. We try category or subcategory slugs
2. We try tags
3. We try tag-groups
The term `hello #my-group` will search for all posts tagged with any of
the tags in the tag group `My Group`
Future work may be introducing a slug cache here or caching it in the table
but the assumption is that the number of tag groups will not be huge