Updated email service to introduce multiple email queues. Queues will now be processed on a priority basis, with urgent emails (e.g. password-resets) taking priority over less urgent emails (e.g. bulk newsletters).
Updated login process so that email address can be optionally used instead of user name.
Minor bug fixes and maintenance (tasks 27249, 26058, 27137) (release linked to Sitesuma Service v 5.36)