TestRTC: User interface revamp.

At the moment available in: https://1b-dot-test-rtc.appspot.com/ (with no turn server).

Issues to solve:
 - Bug report needs fixing to provide a log that can be copy-pasted by the user.
 - Find out if we directly import the polymer elements or we pull a copy into the repository.

But I am leaning towards submitting this as is... and review those issues later.
This is OK, but some devs prefer to use ems for line-height, since they're relative to font size.