This delegates the calculation of the aspect ratio to the browser, and
generally simplifies the code.

Fixes #1156.

This theme takes inspiration from https://discordapp.com/ and builds on
top of custom-properties, making the design more familiar to some users.

In order to change the theme, add the 'theme-dark' class on #conversejs,
you can do it at any point during the lifetime of Converse, either
directly in your HTML or by changing the DOM at runtime.

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.