ForeignNotifications: Move preference check out of constructor and allow force-enable
ForeignNotifications: Move preference check out of constructor and allow force-enable

Instead of checking the cross-wiki notifications preference
in the constructor, move it to a method that other code can reuse.
Also add a parameter allowing foreign notification data to be
inspected even if the user has disabled the preference.

Change-Id: I6600ff27aa5af1737b3a6c3cde586d325886bc86

wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo