Merge partially_unbuttoned and partially_unzipped

I think partially unbuttoned and partially unzipped are both the same concept and distinguishing between the two hurts searchability.
We could alias them both to partially undone, or if there are valid reasons for distinguishing them we can use partially undone as an umbrella tag.

I think it's good to keep them separated if there are more than two characters in the post. Or hell, why not both post #4751537