Default group avatars

[saulmc]

Implements facepile default group avatars per the following logic:

• display up to 4 avatars in bubbles
• arrange bubbles roughly clockwise in descending size
• for 5 or more avatars, display 5th bubble containing "+n"
• exclude the sender's avatar from the facepile, except during initial group creation (I could be wrong about this design direction)
• fall back to the same first letter/0x placeholders as default individual profile avatars
• if there's at least one group member with an image avatar, display that member's avatar in the first/largest bubble position

Note: in dev, some names/first-letters aren't loading as expected. Not sure what this is, but I'm seeing it in the main feature branch as well