Bug 25630: Fix capitalization and terminology for Staff interface system preferences

Bug 25630: Fix capitalization and terminology for Staff interface system preferences

This changes "staff client" to "staff interface" and fixes capitalization
in a couple of cases.

For terminology changes see:
https://wiki.koha-community.org/wiki/Terminology#S

To test:
- Read through the preferences of the "staff interface" tab.
- Staff interface should be used consistently.
- Firefox "PlugIn" was changed to "add-on" reflecting Mozilla's
  own terminology.

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