Bug 21352: Allow plugins to add CSS and Javascript to Staff interface

We should have plugin hooks for the staff interface just like we have for the OPAC as detailed on bug 20181.

Test Plan:
1) Apply this patch
2) Download and install the Kitchen Sink plugin ( v2.1.19 or later )
   https://github.com/bywatersolutions/koha-plugin-kitchen-sink/releases/download/v2.1.19/koha-plugin-kitchen-sink-v2.1.19.kpz


3) Install the plugin
4) Restart all the things if you can ( restart_all if you are using kohadevbox )
   This will ensure the plugin takes effect right away, it should be
   necessary but it won't hurt anything!
5) Load the staff intranet, notice you get an console error log message and the background
   for your staff intranet is now orange ( assuming you've not customized the
   staff intranet in any way )

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
(cherry picked from commit a37637e3)
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>