When browsers with JavaScript disabled try to access Review Board, we

show a warning about the lack of JavaScript, and offer some guidance.

This code is very old, and we single out a solution for Firefox without

considering the modern world of JavaScript in browsers and other

possible extensions.
This change updates this part of the page, using a modern rb-c-alert

and simplifying the information shown. We now show this in the navbar

area, rather than placing it in the main content area (which may be

set to hide until JavaScript shows it, for certain pages). This ensures

it's always visible, and that it's docked toward the top of every page.

Turned off JavaScript in the developer tools and visited a couple of

pages. Saw the new banner.