RTC: [Interface] Change Avatar design

[ingeniumed]

Background

The current design for the avatar that's shown for the collaborators list, in an RTC session looks as follows:

The idea is that, each collaborator has a unique colour border around their Avatar. When they are disconnected, they'll get greyed out and eventually vanish once a set timeout expires.

Proposed Changes

@jasmussen kindly suggested a re-design to get the overall design in-line with other collaborative features in this comment.

The key changes would be:

• Switch from a rectangular box to a circle
• Adjust the margins to get them visually similar with other features like block commenting

FYI @jarekmorawski - can add your thoughts here, so we can have a discussion on the design changes.

[ingeniumed]

Completed in #75595