Refactor password strength indicator code

Splitting handling of the password and password strength confirmation indicators
over-complicates the code. This change exposes all 'events' to both indicators.
Password strength indicator: bugfix

.. don't show a tick when the password is acceptable and the
password confirmation is empty
Debounce password strength indicator

The test_helper change is necessary because the debouncing introduces
timing issues into the JS and the `#has_no_content?` method actually uses
the Capybara wait/retry functionality (and `#has_content?` doesn't).