Apply random colours to usernames

[tdryer]

Apply random colours to usernames in the message list to improve readability (similar to clients like Pidgin).

[averissimo]

I've done this in another app by selecting 26 (arbitrary) different colors and then use the remainder of user_id / 26 as the index for the color.

This gives reproducible colors to each user to keep consistency :)

ps. cache the results to avoid repeating this operation