Rework avatars.

Now we have a concept of an avatar at the server, realm, and scene level
and we have the groundwork for a realm store. New uesrs no longer props,
they get a default avatar. New system supports gender
{male,female,neutral} and {child,adult}.

config.toml

@@ -16,3 +16,13 @@ stale_threshold_secs = 120
 
 # Clear all instance_members on server startup (recommended for single-server deployments)
 clear_on_startup = true
+
+[signup]
+# birthday: "ask" = show field during signup, omit = don't ask
+# birthday = "ask"
+
+# age: "ask" = user selects, "infer" = from birthday, "default_adult", "default_child"
+age = "default_adult"
+
+# gender: "ask" = user selects, "default_neutral", "default_male", "default_female"
+gender = "default_neutral"