people: Enable user_avatar_url_field_optional in webapp.

In 5200598a31, we introduced a new
client capability that can be used to avoid unreasonable network
bandwidth consumed sending avatar URLs of long term idle users in
organizations with 10,000s members.

This commit enables this feature and adds support for it to the web
client.

zerver/views/home.py

@@ -192,6 +192,7 @@ def home_real(request: HttpRequest) -> HttpResponse:
     client_capabilities = {
         'notification_settings_null': True,
         'bulk_message_deletion': True,
+        'user_avatar_url_field_optional': True,
     }
 
     register_ret = do_events_register(user_profile, request.client,