Replace `bleach` with `nh3`

[allanlasser]

In #2160 (comment), @mitchelljkotler flagged:

Claude informs me that bleach is deprecated and nh3 is the replacement

1. We should verify Claude's claim is accurate
2. We should replace our deprecated dependency as suggested if true

[eyeseast]

That's accurate and has been for a while: mozilla/bleach#698

nh3 is built in Rust and should be faster, and it's actively maintained.

[eyeseast]

We should also do this in the DocumentCloud API.