Use stored credentials when checking certain capabilities.

- Since the user had to log in with the ILS to get to the point of changing their password when using this auth method, we can safely rely on the database copy of the username rather than going through the expensive process of communicating with the ILS an extra time.
- Since we are not refreshing the credentials, there is the (fairly small) possibility that the stored password will be outdated. This should not matter for the task of checking the ILS capability, since it is the username that is needed for identifying the user's home ILS in the MultiBackend driver (and in other contexts, this data is not currently used).
- This commit also includes a minor comment improvement and a code simplification.