Bug 25630: Fix capitalization and terminology fixes for Searching system preferences

Bug 25630: Fix capitalization and terminology fixes for Searching system preferences

- Rephrases LoadSearchHistoryToTheFirstLoggedUser system preference
- Consistently use "staff interface"
- Add ending punctuation to some preferences

To test:
- Read through the Searching system preferences
- Verify consistent use of staff interface
- Verify LoadSearchHistoryToTheFirstLoggedUser description is
  improved

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>