Make the in dialog set-password flow work with requiredEmail.

* In set_password.ejs, only show the "cancel" button if not setting the initial password for a required email.
* Update required_email.js to handle the set-password in dialog flow.
* Clean up required_email.js so that it is easier to follow.
* Rename the "add_email_submit_with_secondary" message to "stage_email" - much clearer.
* Show a message whenever resetting the account password.

issue #1638
issue #1580