Support paragraphs breaks in BB comments

I just noticed something we might want check on a later stage, which is that extra empty lines are discarded in D2L and Moodle, but they are preserved in Blackboard.

With "extra" I mean that it removes one empty line when saving, which means that, if you add more than one, it keeps one less every time, but if the same comment is submitted more times, eventually all empty lines are lost.

[Grabación de pantalla desde 2023-12-13 09-58-19.webm](https://github.com/hypothesis/lms/assets/2719332/9d127bca-9ec6-4cf1-b6bc-c2232d301f14)

_Originally posted by @acelaya in https://github.com/hypothesis/lms/pull/5913#pullrequestreview-1779149837_