Redesign (like main website)

Demo is at http://73.sympy-live-tests.appspot.com/

Changes:

- Unified desktop/tablet/mobile site (mobile site now redirects to homepage)
- Get rid of DS site (redirects to homepage)

Also see: https://github.com/sympy/sympy-web-static/pull/1

- [ ] Fix width of the `>>>` box.