settings: Rename headings in notification settings UI.
settings: Rename headings in notification settings UI.

This commit renames 'desktop' and 'mobile' headings in
notification settings UI to 'desktop message notifications'
and 'mobile message notifications' respectively.

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