Bug 20181: Allow plugins to add css and javascript to OPAC

Bug 20181: Allow plugins to add css and javascript to OPAC

A number of Koha plugins have been written that enhance Koha's public catalog. These plugins often make due to adding css and javascript to the various opac system preferences. It would be nice if the plugin system had hooks so plugin developers could add code the the head block and the area where we include javascript in the opac template.

Test Plan:
1) Apply this patch
2) Download and install the Kitchen Sink plugin ( v2.1.12 or later )
   https://github.com/bywatersolutions/koha-plugin-kitchen-sink/releases/download/v2.1.12/koha-plugin-kitchen-sink-v2.1.12.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 opac, notice you get an alert message and the background
   for your opac is now orange ( assuming you've not customized the
   opac in any way )

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>