Bug 27607: Add the ability to compare patron records during merge process

Bug 27607: Add the ability to compare patron records during merge process

This patch adds an option to compare two patron records during the
patron merge process. It can't work with more than two patrons, so the
option only appears if one has selected only two patrons to merge.

This patch also adds links to the patron names for opening the patron
record in a new tab.

To test, apply the patch and perform a search for patrons in the staff
client Patrons module.

 - Select two patrons in the list of search results and click "Merge
   selected patrons."
 - On the merge patron records page, confirm that the two patron names
   are links which, when clicked, open the patron details in another
   tab.
 - Click a radio button to select the patron whose record will be
   the basis of the merge. The "Compare patrons" button and "Merge
   patrons" should go from disabled to enabled.
 - Click the "Compare patrons" button. A modal window should display
   highlighting the differences in some selected values in the patron
   record. The patron you selected as the basis for the merge should
   appear on the left.
 - Close the modal and select the other patron as the basis of the
   merge. "Compare patrons" again. The other patron should now appear in
   the left pane of the comparison.
 - Test that the "Merge patrons" button in the modal window works
   correctly to submit the merge form and that the correct patron is
   used as the basis for merging.
 - Repeat the same process above but select more than two patrons for
   merging. The "Compare patrons" button should not appear.

Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>