settings: Use variable for notification sound element.
settings: Use variable for notification sound element.

This commit changes the click handler for playing
notification sound to use a variable instead of
directly using the element id such that we can
use the same code for realm-level settings also
by just setting the variable accordingly.

kou/zulip,zulip/zulip,rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,eeshangarg/zulip,rht/zulip,eeshangarg/zulip,andersk/zulip,rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip,kou/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,eeshangarg/zulip,zulip/zulip,kou/zulip,andersk/zulip,kou/zulip,andersk/zulip