Replace bleach, possibly with nh3

[rolandgeider]

The html5lib, which bleach depends on, is deprecated (mozilla/bleach#698). A possible alternative that seems to do what we need is NH3: https://github.com/messense/nh3

Alternatively implement #1240

[eyJhb]

Maybe also add a tracking issue for moving away from django-bootstrap-breadcrumbs2, as the requirements.txt state should happen.

[rolandgeider]

-> #1845 :)