(stable) Fix Content-Security-Policy headers on "Email Login" page

Summary:
In D20100, I changed this page from returning a newPage() with a dialog as its content to returning a more modern newDialog().

However, the magic to add stuff to the CSP header is actually only on the newPage() pathway today, so this accidentally dropped the extra "Content-Security-Policy" rule for Google.

Lift the magic up one level so both Dialog and Page responses hit it.

Test Plan:

• Configured Recaptcha.
• Between D20100 and this patch: got a CSP error on the Email Login page.
• After this patch: clicked all the pictures of cars / store fronts.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20163