Bug 15327: Minor tweaks to Bootstrap modal handling on Staged MARC management page

Bug 15327: Minor tweaks to Bootstrap modal handling on Staged MARC management page

This patch makes a couple of minor corrections to the template for
staged MARC management. The MARC preview is now formatted using the
"html" option, and some obsolete Greybox-related code has been removed.

To test:

1. Apply the patch and go to Tools -> Staged MARC management.
2. Click any staged MARC file containing multiple records.
3. Click the title in the "Citation" column and confirm that a Bootstrap
   modal is triggered showing the correct data.

Signed-off-by: Hector Castro <hector.hecaxmmx@gmail.com>
Modal looks much better. Works as advertised

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit 23c7de2bc7af7d2d8bf725e536f665bf78116065)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>