Remove selectize. Flat/square elements.

@marcusramberg: You can try this out on demo.convos.by.

I can't remember your idea about the right-side buttons in the navbar, but at least all the sidebars have equal width now...

The changes are:
* Remove selectize.js
* Use CSS to animate sidebars
* Different layout for input field (dependent on screen width)
* Simpler _chat.scss
* All sidebar-togglers are always visible
* Flat/square form fields
* LOC diff: +108  −3,627