Check for existence of similar emails

Warn the user if selections are made that are similar "enough" to an existing email.

For instance, if the user selects:
- action = new
- type = post

... then warn the user that an email already exists for those selections and ask, "Are you sure you want to create a new email?"

If ALL data matches an existing email, then stop and tell the user that an email with those settings already exists.
