Server Notices – room_avatar

This issue has been migrated from [#12831](https://github.com/matrix-org/synapse/issues/12831).

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Hey

I just set up `server_notices` and found that the room avatar would be a better option than `system_mxid_avatar_url` or just having both.

Or is it already available but not [documented](https://github.com/matrix-org/synapse/blob/develop/docs/server_notices.md)?

Something like this:
```
server_notices:
system_mxid_localpart: server
system_mxid_display_name: "Server Notices"
system_mxid_avatar_url: "mxc://server.com/oumMVlgDnLYFaPVkExemNVVZ"
room_name: "Server Notices"
room_avatar: "mxc://server.com/oumMVlgDnLYFaPVkExemNVVZ"
```

Thanks!