Remove PKs from tag fixture now that we have natural keys for them

[glennmatthews]

Relates-to #1574

What's Changed

Add slug as a natural-key for Tag. Though it occurs to me we might need to revisit this depending on what we decide in #2312.

This is a PR atop #2281 and #2360 since the latter allows me to remove the UUID PKs from the Tag fixture file.

[glennmatthews]

@lampwins Should I change this (and/or the Status natural-keys support as well) to use name instead of slug as the key?

[lampwins]

@glennmatthews because TagBase.name is unique, I would say yes because name is already a true natural key.

Incidentally, in double checking that in the taggit code, their handling of the slug field is a prime example of one of the reasons I truly believe slugs are almost never natural keys. I'll post my thoughts on #2312 :)

[glennmatthews]

It might make sense to target this to develop instead, actually, since natural-keys for Status were added in 1.4.1. Thoughts?