When we added the ability to map Student ID and Display from a LDAP
server, we completely forgot that the Student ID can be specific for
the institution. In fact, if we don't store the Student ID in
usr_institution we are not able to search for the user in the
institution users page using the Studend ID.

Full credit goes to Patrick Pollet for discovering the bug and
proposing a fix for it.

Change-Id: I545bbcb5addf837364ed1178be340d170d4114e3
Signed-off-by: Iñaki Arenaza <iarenaza@mondragon.edu>