Library card fixes and improvements (#1257)

- Security: do not pass the user's existing password back into the template; instead, leave the password field blank and check credentials only when it is changed.
- Bug fix: when the user updates an active library card, update the user table as well as the library_cards table.
- Bug fix: when the active library card contains invalid credentials, allow the user to either log back in with good credentials or switch to a different card.