Bug 27592: Link audio alerts to corresponding preference and back

Bug 27592: Link audio alerts to corresponding preference and back

This patch adds links from the audio alerts page to system preferences
and back.

To test, apply the patch and disable the "AudioAlerts" system
preference.

- Log in to the staff interface as a user with permission to access
  audio alerts and system preferences.
- Go to Administration -> Audio alerts.
- You should see an alert, "Audio alerts are disabled."
  - Test the link to system preferences. It should take you a
    preferences search which returns only "AudioAlerts."
- On the system preference page, test that the link to audio alerts
  works correctly.

- Disable audio alerts again.
- Log in to the staf interface as a user with permission to access audio
  alerts but WITHOUT permission to access system preferences.
- Go to Administration -> Audio alerts.
- You should see an alert, "Audio alerts are disabled."
- There should be no link to system preferences.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>