[Improvement] Write own update databse function

We rely on Flask Migrate to update database schemes.

This is not good, as it varies from the local installation and not just from version to version. Partial updates during development lead to a different update script, e.g. for production systems.

If we do the migration by hand, we can introduce each change and combine it with a check (if table entry exists or if the name has changed). This is more flexible, e.g. if we want to rename table entries (and not delete them and reintroduce a new field).

As we already depend on SQLAlchemy and Alembic, this might not be too difficult to do. We would check the state - e.g. field exists or field has an old name - and carry out the appropriate action. If we do this sequentially, we can always update old database schemes independent of the current state.