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 

Namely:
- Only match messages which start with exactly "/me ".
- Simplify the match, checking a string instead of a complex regex.
- Always remove the first four characters in the case of a match, to not
leave an extra leading space.