luci-theme-bootstrap: Allways show scrollbar

luci-theme-bootstrap: Allways show scrollbar

With this addition to the body tag, on desktop browsers the vertical scrollbar
is always visible and the content will not be shifted by changing tabs (like
at System/Software when the update tab has no to little content).

I couldn't see any negative side effects with Chromium browsers or Firefox
under Windows/Linux and Android, but I don't have access to Apple devices
for tests.

Signed-off-by: Thomas Schröder <tschroeder_github@outlook.com>