Bug 18122 - Audio alerts: Add hint on where to enable sounds

Bug 18122 - Audio alerts: Add hint on where to enable sounds

This patch adds a hint to page Home > Administration > Audio alerts
and related help page about where to enable the sound system
(syspref 'AudioAlerts').

To test:
- Apply patch
- Disable syspref 'AudioAlerts'
- Go to Home > Administration > Audio alerts
- Note hint below title 'Audio alerts'
- Enable syspref 'AudioAlerts'
- Go to  Home > Administration > Audio alerts
- Hint is no longer visible (since audios are enabled)
- Open Help page (top right)
- Verify that hint on help page makes sense.

Amended to add alert class as requested by comment #2 2017-02-21 mv

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>