* Fix failure to create category with deleted name

I'm not 100% familiar with the design of the data model so may have
misinterpreted what's going on here, but how I read this:

- The tombstone flag is used to soft delete categories.
- When creating a new category, duplicate names within a group are
  prevented.
- When checking for duplicate names, the tombstone flag was unchecked
  which meant deleted categories were falsely blocking creation.

I had a look at category deleteion logic to verify that simply including
the tombstone flag in this check is sane and see that there's a category
mapping being maintained to redirect one category to another on
deletion. So from what I can tell the correct behaviour here is to allow
a new category with the previously deleted name, rather than to revive
the old category (to preserve that old mapping lineage).

* Add release note

* Add regression test