fix: use deterministic avatars for channel members

[oliverlaz]

Goal

Under some circumstances, the avatars of the members in a channel were selected randomly and in unpredictable fashion.
This behavior can be annoying sometimes especially when testing the multi-user chat channels.

With this PR, the selection of avatars is made a bit more deterministic and relies on the user_id parameter.

Implementation details

• we calculate a hash value for the given user_id (should be stable enough for testing/demo purposes)
• based on the hash value, we assign an avatar to the user from our list of predefined avatars