Use per-server avatars

[DuckMasterAl]

Is your feature request related to a problem? Please describe.
The bot does not display per-server avatars to users

Describe the solution you'd like
The bot displays the per-server avatar for a user if applicable.

Describe alternatives you've considered
No alternatives have been considered.

Who will this benefit
This feature will apply to a larger group of users once per-server avatars are public, but for now, it doesn't apply to most users.

Additional context

(Username information has been censored)

[fourjr]

Not supported in discord.py, see Rapptz/discord.py#7054

Tagged to #2990