Bug 8085: Rename 'Reserve slip' to 'Hold slip'

Bug 8085: Rename 'Reserve slip' to 'Hold slip'

This patch changes all occurrences of 'Reserve slip'
to 'Hold slip' in the sample notices files and
in one of the system preferences.

To test:
- Make sure the sample_notices.sql are correctly
  changed.
- Verify the description of the SlipCSS system
  preference has been changed correctly.

Checked the working in system preferences and re-loaded notices from the
updated English sample notices SQL. Changes look correct.

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>