Bug 16793: Use Font Awesome for arrows instead of images in audio_alerts.tt

Bug 16793: Use Font Awesome for arrows instead of images in audio_alerts.tt

This changes fit with bug 16148 for marc modification template management

To test
-Apply patch
-Goto Admin->Audio alerts
-Verify that green arrows have disappeared now
 in blue Awesome Icons

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
https://bugs.koha-community.org/show_bug.cgi?id=16794

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit 90da601783e9f368939d41affb250289dd10204b)
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>