Set reference to roster contact on the message and chatbox object and
listen for changes to the nickname.
Currently, because chat boxes are fetched and initialized before the
roster, messages and chats are repainted with the correct display name
only later, causing a "flash" effect.
Ideally we would only initialize the chat boxes after the roster
contacts have been fetched, but this is currently not easily possible
because we need the control box to render before everything else.
Until now, we've only been explicitly documenting the `_converse.api`
namespace and only considered the methods under it as forming the API
contract (which determines how we do semver releases).
It appears as if we've reached a point where trying to keep everything
under the `_converse.api` namespace no longer makes sense. Certain
methods are applicable to particular models and trying to shoehorn them
into the `_converse.api` namespace seems clunky and non-intuitive.
I've therefore decided to slightly refactor `sendMessage` to let it take
two simple parameters and to document it with JSDoc so that it's
presented as an API method, albeit only available on a chat model.
updates #1496
updates #1504
