[Fix #128] Styling fix for main layout.

Two problems solved. The one that generated #128 was a *D'oh!*-level
basic CSS issue; fixed blocks (e.g. navbars) need to have their space
accounted for by offsetting non-fixed, adjoining elements.

Also got around to fixing a change between Bootstrap 2 and 3, where the
'alert-error' style is now called 'alert-danger'. Including Bootstrap's
JavaScript also helps.

RSpec: 762 examples, 0 failures
RuboCop: 175 files inspected, no offenses detected