Improve user notes about salr-redux

[astral-sa]

Next thing I'm looking at working on is adding the user post background coloring to the user notes.

I'd also like to add a related feature from legacy Firefox SALR - individual user avatar toggling.

[astral-sa]

I've got user post background coloring done locally.

A future improvement would be to also have the backgrounds of highlighted folks' quotes colored.

Would need:

• add username to userNotes object (can update them upon creating the note and, after that, if/when they are seen in a thread)
• reverse lookup table created at runtime to go from username->userid which also probably needs to be updated when new info is added from the above

Another future improvement would be to add a true color picker to the color options for the notes.

Can definitely wait on these until after the intended "Only loop through posts once" changes