Slightly repurpose Notifier::getEmail
Update Notification::issue to only send instant notifications
Some update of language strings to make things clearer
WeNotif:;getNotifiers now returns null if invalid notifier is passed
Rename WeNotif::scheduled to WeNotif::scheduled_prune in order to avoid confusion
Add periodical e-mails scheduled task
Make sure the periodical period is positive
Actually load the plugin template before handing out the e-mails