Add Not Now option to save password? popover

We lost this option in the transition from info bars to the popover-based UI. If you don't want to make any decision on whether or not to save the password, then your only option is to click outside the popover, which causes the password save request to stick around indefinitely.

Permission requests have the same problem, but I think I'm OK with that as they occur less-frequently, and it's easy to change permissions later if desired, and it's usually easy to decide whether to allow or deny a permission request. But for passwords, you want to click "Not Now" frequently, e.g. whenever creating an account for a new service (since passwords are associated with form elements, and you surely want to save the password for the login page, not the account creation page). Since this is a relatively frequent workflow, we should make it easier.

Fixes #2291 (closed)