Fix highlights order in notebooks

[kuosandys]

Fixes an issue with highlights display order in notebooks, as described in https://github.com/omnivore-app/omnivore/issues/3789:

• highlights with the same highlight position (i.e. in same line) are now sorted by their location within the line
• the same highlight sorting was performed in a few places - the sorting function been refactored to one common location

I also see that there's a new Highlights view in the works - I didn't want to increase the scope of this bug fix PR to include feature work, but let me know if any help is needed there!