LDAP: prefer description over departmentNumber. (#399)
LDAP: prefer description over departmentNumber. (#399)

In #398, we introduced parsing a (new) LDAP field description, which substitutes the old
`departmentNumber` field for new students.

We have however noticed that the `departmentNumber` field exists for (only a few) of the
new students, however with uninformative input such as "Student".

To ensure smooth operation, we should thus check the description field first, as it is the
new official field with the information we need, and only fall back to the departmentnumber field
if it is missing.
amiv-eth/amivapi,amiv-eth/amivapi,amiv-eth/amivapi