Bug 30288: Provide links to OPACUserJS and OPACUserCSS in tools

Bug 30288: Provide links to OPACUserJS and OPACUserCSS in tools

This patch provides easy access to the OPACUserJS and OPACUserCSS
system  preferences in the Additional Contents (News and HTML
customisations) sections.

To test:

1. In the staff client, go to Tools -> News. Notice there is a button to
   add a New entry. Confirm this button is also there when you go to
   Tools -> HTML customisations.
2. Apply the patch and refresh the page.
3. Confirm there are now two new buttons to take you to OPACUserJS and
   OPACUserCSS. Confirm these buttons take you to the correct system
   preference.

Sponsored-by: Catalyst IT
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Bug 30288: (follow-up) Add permissions check

This follow-up wraps the system preference links in a check for
"parameters_manage_sysprefs" permission so that the links don't display
to those without permission to access system preferences.

Signed-off-by: Aleisha Amohia <aleishaamohia@hotmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>