Add CSS classes to differentiate between incoming and outgoing messages

This is mainly a testing tool, which makes a mode where all HTML bugs
which could slightly mess up the rendering will trigger proper errors in
the browser.

Fixes #1131.

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.