Re-focus the password field after clicking the show button.

fixes #457
Only show the "must be at least 8 characters" text on the /signup page when the user focuses the password element.

The help text for change-password and complete-reset-password are always displayed because there are two password inputs.

fixes #382
Keyboard accessible password show buttons!

fixes #414
Add the missing aria-controls target

* in change_password.mustache and delete_account.mustache
Merge pull request #476 from mozilla/issues-382-414-457-password-smoothness

Make the password fields more usable.