Migrate to AlekSIS app

Turn TIC-Desk into a AlekSIS app. Everything that can be generalised should be turned into AlekSIS core functionality or generic apps, and everything that is really teckids-specific should be turned into a AlekSIS app as well (but not as an official app). This should be very well-planned before starting.

Doing so, some things will fundamentally change:

• Primary data management will be in PostgreSQL, with LDAP import and export to be added
• LaTeX printouts will be turned into paper.css
• English will be the primary language, and a German translation will be added

TIC-Desk itself can stay in this very repository and the migration can be done here.