Bug 15694: Add aliases for date/time last modified

 Bug 15694: Add aliases for date/time last modified

This patch will add indexes for Date/time-last-modified.

To test:

1. apply patch
2. reindex
3. search for dtlm:DATE and date-time-last-modified:DATE
4. confirm that you get results

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

Signed-off-by: Frédéric Demians <f.demians@tamil.fr>
  I confirm Hector signing-off. A simple Zebra server restart suffice to get
  working the searches on date-time-last-modified and dtlm.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>