- Removed password encryption of the key.
  It never properly worked and bloated the build through extra dependencies.
- Store the key and instance tag on the ChatBox model.

"TypeError: this.sendConfiguration(...).then is not a function" when a room is created

When returning chat boxes via the API.

We can do this now because these API methods are only available to
(whitelisted) plugins.

Otherwise `require('converse-core')` can be called, exposing the inner converse
object.

Much smaller library.
No dependence on jQuery.

Updates #779

This reverts commit e6360722.