Webcams: allow using avatars as webcam container placeholders

[prlanzarin]

Is your feature request related to a problem? Please describe.

A webcam container placeholder stands for the intermediate video-list-item UI that is shown while the remote stream hasn't been loaded and played yet.

Our current webcam container placeholder is a black background with the user's name. A step forward from the old loading spinner.

We'd like to improve it further, though, by using avatars of placeholders.

Describe the solution you'd like

• If an avatar image was provided by the frontend, use it as the placeholder
• If an avatar image was not provided, consider using a default avatar (design TBD, but it could be in the same vein as the user list's avatar)

Additional info

Blocks #13844

[prlanzarin]

cc @antobinary

[prlanzarin]

This issue originally had the following as a reminder of a spin-off feature:

An optional mode (behind configuration/API flags) could be added where every user has a webcam container with a placeholder even when they aren't sharing a camera.

I've edited that out in favor of #13844.
This issue blocks #13844.