General topic: How to deal with changes to the DB

[renatobellotti]

A policy is required for interaction with the database:

• If multiple changes are applied to the DB, should we do them in a transaction or one by one? For critical parts, e. g. the payment processing code, this can lead to issues (like in HS2017, where a too long payment comment crashed all payment transactions for one month).
• What parts of the code are considered critical?
• What to do if an error occurs?
  • Silently drop the actions?
  • Let Sentry know (originally proposed in #149)?
  • Print an error message to the user?
  • Write an email to the admin?