Better form/validation handling for RedHen settings associated with Drupal user accounts.

Just found another potential issue, though I'm not sure if it's a bug created by this change or a pre-existing bug we'd not encountered.

If email mirroring is turned on for RedHen Contacts and Drupal User Accounts while RedHen Email Addresses are not required, you get some strange behavior.

To recreate:

• Turn off the "Require contacts to have a valid email address" setting.
• Leave "Mirror RedHen Contact email to Drupal user" enabled.
• Create a RedHen contact w/o an email address.
• Create a Drupal user account (email address required of course).
• Connect the RedHen contact with the newly created Drupal user account.

Result:

• The RedHen contact is not updated with the Drupal user account's email address.
• Adding a new email address for the RedHen contact overrides the Drupal user account's email address.

Solution:

Add validation and UI affordances to the RedHen configuration screen to make the "mirror" option only available when RedHen Emails are required. This "mirror" option should also only be available when RedHen Contacts are being linked to User Accounts.

This ticket originated with: #2023893: E-mail field should better collapse options fieldset.