Feature request: Random avatars for unknown numbers

[jsha]

• I have searched open and closed issues for duplicates

Bug description

When participating in group conversations, if I don't have names in my contacts for most participants, they are all represented by the same grey "#" icon, making it hard to tell apart different people.

Proposal: Unknown numbers should be represented by a random color, and possibly a random shape as well for extra distinguishability and accessibility for color blind people.