Set icon based on the avatar id

When the room avatar is an icon that should be provided by the client
the avatar id starts with "icon". The icons follow the same name
currently used in the web UI ("icon-public", "icon-contacts",
"icon-file", "icon-password", "icon-changelog" and "icon-mail") so the
name can be directly used.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>