Bug 11772: Show warning on About page:System information if no active currency is...

Bug 11772: Show warning on About page:System information if no active currency is defined

This patch displays a warning on the About page when no active currency
is defined.

Test plan:
1)
Make sure that no currency is defined as active
(Home › Administration › Currencies & Exchange rates › Currencies)
2)
Go to Home › About Koha > System information
Result: No warning issued.
3)
Apply patch and reload About page
Result: Warning issued.

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>