2073606 - Add a setting to allow website notifications while Firefox is closed

Status: NEW

(Firefox :: Settings UI, enhancement, P1)

Description

[Hanna Alemu [:hanna_a]]

Add a checkbox on the Notifications page, "Allow website notifications when
Firefox is closed", unchecked by default, with the helper text "Your OS
notification settings also apply." It sits below the list and above the
"Block new requests" checkbox.

The checkbox is the user's own switch, app.backgroundNotifications.helper.enabled.
Write it through PushNotificationHelper.setEnabled() rather than setting the pref
directly. The separate gate app.backgroundNotifications.helper.available is the pref Nimbus owns, so the
two must not be confused. The helper runs only while both are true, and its status
persists across restarts.

Render the checkbox only where closed-browser delivery is available, meaning the
Nimbus gate is set. Where it is not, the checkbox is absent rather than disabled. The gate is declared Windows-only

Ticking the checkbox changes no per-site value; it changes what those values
mean. Every site reading 'Always' becomes eligible for closed-browser delivery at
once, which is every allowed site until one is set to "When Firefox is open",
the per-site opt-out. There is no per-site enrolment step.

ENABLE_CLOSED_BROWSER_NOTIFICATIONS and DISABLE_CLOSED_BROWSER_NOTIFICATIONS
write the same pref from messaging, so the checkbox has to reflect a change made
while the page is open.