Reset active directory passwords

Hi Todd,

This is the second feature. It builds on top of the "create_active_directory_accounts" feature.

This is how it works:

There is a new "Forgot password" link in the login page. When a user clicks this link, he is prompted with a form to validate his cmu email address and his personal email address.

Once the user is authenticated, a whiteboard sends a link to him for password reset. The link expires after a certain time.

When the user clicks the clink, he is directed to a page where he can create a new password. The new password is encoded and sent to active directory over SSL.

This pull request is for code review. Otherwise Jazz will inform us when he is ready to roll out the new setup.