[UX] Change the password reset form to use a single password field + show/hide toggle

This was brought up in https://github.com/backdrop/backdrop-issues/issues/4281#issuecomment-578...

@stpaultim

I notice that we only ask for a new password once when a user creates a new account. We only ask for a a new password once when an authenticated user chooses to change their password.

Should we ask for it twice when a user uses password reset to create a new password.

I don't feel strongly about this. But, it seems like Backdrop CMS has already made a decision that we don't want to require the double password thing. So, why not be consistent.

@indigoxela

Good point. As we're already working on it, why not switch to a single field?

  $form['pass'] = array(
    '#type' => 'password',
    '#password_toggle' => TRUE,
    '#password_strength' => TRUE,
    '#required' => TRUE,
  );

...there's a concern: Changing the form field type could be a contrib-breaker, for instance for passphrase_policy module. This has to be evaluated, though.

Setting the same field to required is a task for a simple bugfix release, changing the form item type requires more evaluation and probably isn't appropriate for a bugfix release.

@stpaultim I'd rather recommend to create a new issue for the form field type change.