Notifications: Conditionally load action and screen functions.

This commit conditionally loads action and screen function code for the
Notifications component, utilizing the 'bp_late_include' hook introduced
in r11884.

Previously, we loaded these functions at all times, which is unnecessary
when a user is not on a BuddyPress notifications page. Now, we only load
this code when needed.

This commit also moves over the unused bp_notifications_screen_settings()
function from bp-notifications-screens.php to
bp-notifications-functions.php. This function is a candidate for future
deprecation.

See #7218.