Bug 31413: Set focus for cursor to Selector when adding a new audio alert

Bug 31413: Set focus for cursor to Selector when adding a new audio alert

This patch adds a step to the process of displaying the new audio alert
form so that focus will be set to the first field when the form has
finished appearing. The typical "focus" class only works on elements
which are present in the initial DOM when the page is displayed.

To test, apply the patch and go to Administration -> Audio alerts.

Click the "New alert" button. When the form appears cursor focus should
be on the "Selector" field.

Also test the edit process: When editing an existing alert there should
not be automatic cursor focus.

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit 7c8434553a29b177b879f958c59487cf854c3c13)
Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>
(cherry picked from commit 5e593d098dab2f33a7a7ade03b7af64801765d8b)

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
(cherry picked from commit b0a16847fedcf8edc010247e8898ddfae90a1b3a)
Signed-off-by: Arthur Suzuki <arthur.suzuki@biblibre.com>