Refactor reports worker for new reporting flow

[Donnype]

TODO:

• Use reference_date as valid_time filter for all (asset) reports (should we use now for the valid time of reports we generate now?)
• Check all references to parent/child reports in the runner
• Fix and refactor report naming
• The ReportRecipe is now nullable, but it is a part of the pk of a Report, so this will not work.
• QA generating (concatenated) reports
• Fix and refactor aggregate report naming
• QA generating aggregated reports
• Set the right reference date (from the scheduler task?).

Questions:

• Are we allowed to set/change the date_generated of a report?