Bug 26015: Terminology: Use staff interface instead of staff client

Bug 26015: Terminology: Use staff interface instead of staff client

We have agreed to use 'staff interface' instead of 'staff client'
and similar terms. This fixes the template files where staff client
appears:

To test:
- Check Administration > Patron categories
- Check the title of the staff interface start page in your
  browser tab
- Check the description of hte catalogue permission
- One more: this changes the message shown when someone managed
  to log in as the database user - as we no longer allow that, I am
  not sure if it can be triggered.

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@helsinki.fi>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>