Send Email to contacts - give warning and stop user from continuing if recipient count is greater than limit of 50 BEFORE the form is loaded

Currently if you queue up > 50 recipients for a regular "Send Email" task (not CiviMail) - you can fill in the Email form and you don't get the warning until you click Send. At this point you've lost your email text (it doesn't even save updates to a msg template) and have to start again.

To prevent this frustration, we should give the "too many recipients" warning and redirect back to search - since we should be able to detect up front that the number of selected recipients is greater than the max allowed.