Email sending queue notification.

[corsacca]

Note: Emails are created in the queue to send.
So cron needs to be running to process sending emails.
This is because the system can not handle sending 100s of email before the 30 second timeout.
We could say: Emails queued to be sent, instead of emails sent.

Also add this notice in the list bulk send sections.

When DiscipleTools/disciple-tools-theme#2292 is done we can link to it to see queue.

Originally posted by @corsacca in #130 (comment)