Browser JS style fixes

- Spaces not tabs
- Fix whitespace
- Use 'on' properly to get clickable element
- Obey JSLint
Use JS to show browser warning

It doesn't really work (or erroneously works, since your browser might
be good enough to avoid being shown the notice) when JS is unavailable,
so it should only ever be shown using Javascript.
Merge pull request #317 from alphagov/progressively-enhance-browser-warning

Progressively enhance browser warning