change password UX cleanup

* If the passwords are the same, print an error message.
* Hide any error messages when a success message is printed.

fixes #336
fixes #343
Add the password rules below the new password field.

fixes #342
Fix the duplicate show-password element ids.

* change one to 'show-old-password', the other to 'show-new-password'

fixes #340
Cleanup of the change password errors.

* Change the "passwords are the same!" error to "old and new passwords must be different"
* Hide error messages on form submission.
* Hide the form if the password change was successful.
Merge pull request #360 from mozilla/issues-336-343-change-password

change password UX cleanup