- Add support for switching ephemerality after message creation
- Move more methods from ChatBox and ChatRoom to utils/stanza.js
- Rename 'ephemeral' to 'is_ephemeral' since it's a boolean
in which you can set user affiliations and roles.
Also, let getAffiliationList return an Error instead of `null` if you're
not allowed to fetch a particular affiliation list.
