Turn school information into model extension

The fields o, ou and l shall be turned into extra fields injected into AlekSIS' Person mdeol, preferrably with new and better-fitting names.

created merge request !57 (closed) to address this issue

mentioned in merge request !57 (closed)

Will be done in !55

mentioned in commit 3ff2046d

mentioned in merge request !55

changed milestone to %TIC-Desk 2.0