Support a "letter" as the Group avatar

[joaquimrocha]

Currently we have group avatars as simple colored circles. While this is fine for many users, it is a problem for color-blind users, and in general it's easier to identify groups if we add a letter/symbol to them.

This requires:

1. Another field for writing the unicode symbol/letter
2. Computing a symbol's color that contrasts well with the avatar color (this can be done automatically using something like https://github.com/moroshko/accessible-colors ).