Re-enable form submit when password strength check not needed

[tonyin]

At the non-admin user "Account Settings" > "Change email address" page, the "Update email" form submit is disabled due to the "check_password" macro (line 5) not finding the "new_password" {{ field }} passed to it from "manage.html" (line 52).

A short-term patch would be to add a line to re-enable the form submit if the element does not exist.

Screenshot of issue attached.