Improve color-coding for user disambiguation

This is an attempt to provide user color dynamically based on the current kirigami theme (background and text color from the theme). Coefficients are handpicked so it could be improved.

I would be happy to get feedback. Feel free to criticise/close 🙂

See also: https://github.com/quotient-im/libQuotient/wiki/User-color-coding-standard-draft-proposal

Here're the screenshots. The left side is the current master state. The right side has updated user names and avatars without pictures according to the current text and background colors.