Greatly simplify the nick color generation and properly detect dark themes.

Instead of randomly adjusting colors based on some seed colors we now hash a user's name and use the first 2 bytes of the digest to determine the hue to color their nick. We then adjust it according to the background color.

This isn't perfect as you can see in the screen shot, but there are some tweaks we can do later to account for that. I skipped them for now as this gets us 90% of the way there with like 50% of the work.

Testing Done:
Compiled and ran locally with Adwaita, Adwaita-Dark, and Arc-Dark themes.

Bugs closed: PIDGIN-17463

Reviewed at https://reviews.imfreedom.org/r/423/