show version info in login dialog

Send presences according to XEP-0319: Last User Interaction in Presence

Won't be included in the repo, but is useful in certain cases, for
example when embedding Converse in a web component, the fonts need to be
loaded in the main document.

* Return the promise, so that we can wait.
* `sendContactAddIQ` now returns a promise and doesn't take a callback
* Don't call `destroy` on a contact that's already been removed.

fixes #1033