Unifying the "we sent email" messages across all locations.

* Unifying both the message (with slight variation for current action) and the styles across dialog and main site.
* Adding a throttling message to the signup page.
* Adding a couple of page helper functions to take care of showing notifications.
* Adding the ability to go "back" in all of the main site email sent notifications in case of error.
* When going "back" after sent email, ensure the stored email is restored.

close #579
close #577