luci-theme-bootstrap: fix overlay display failures

Fixed overlay display failures for users on mobile, or users who booted
up in safe mode XD. Noticeable when vertical or horizontal real-estate
is too small for the displayed overlay.

This failure is most evident when you "edit" an interface, or display
your unsaved changes.

When content is wider than the screen view, the table hugs the (widest)
content maximally, and the overlay window becomes scrollable.

Disabled -webkit-overflow-scrolling, left, and right, because both FF
and Safari disable them as invalid anyway.

Signed-off-by: Paul Dee <systemcrash@users.noreply.github.com>
[Merge duplicate modal declarations, remove not applicable modal
 flexbox properties]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>