HB: Add private schools

[cyroxx]

In the Schulwegweiser, private schools are not listed. Those can be found in:

• Private allgemeinbildende Schulen
  https://www.bildung.bremen.de/privatschulen-3829 and
• Private Berufsschulen
  https://www.bildung.bremen.de/privatschulen-105554

[k-nut]

As those pages contain only links, we probably want to take the data (for this school and all other schools) from the spreadsheet linked in #56 instead.