Bug 30952: (follow-up) Fix style of floating toolbars

Bug 30952: (follow-up) Fix style of floating toolbars

This patch makes some changes to floating toolbars, both hc-sticky
toolbars and DataTables floating toolbars. The changes fix some bugs in
the display (incorrect z-index, unwanted margins) and proposes a
different style for .searchheader-style toolbars (e.g. patron search
results).

To test, apply the patch and rebuild the staff interface CSS. Test pages
with various kinds and combinations of floating toolbars, e.g.:

- Administration -> System preferences
- Patrons -> Patron search results
- Administration -> Libraries
- Catalog search results

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>