Overflow on body was set to hidden, likely because of
various tricks used to hide elements off screen in the
SPA. Overflow hidden also has the benefit of hiding
scroll bounce when using a MacBook touchpad. The
welcome page is the exception that needs scrolling,
so style welcome page to scroll.

A couple others had made pull requests to addrses this issue
but there hasn't been follow up.

All z-indexes found in css files have been moved into css
variables. If the z-index is used only once, the variable
name will be the same as the selector it is used in. If
the z-index is used multiple times, then the plain name
of $zindex# was used. This allowed a more confident
moving down of the toolbar so that the new modal dialog,
with z-index 500, could display on top of it.

#1436

This reverts commit 93648f361b.