Add feature to delete a run

[danielhuppmann]

We need to add a feature to delete a run, and there are two strategies for implementation.

• A "true" deletion: delete all entries from the RUN table, iamc datapoints, measurands, meta indicators.
• Hiding a run for non-admin users ("soft delete"): this would be a nice option to allow modelers (non-admins) to remove a run but still have the option to easily restore it

The main rationale for this distinction is that I do not want to allow non-admin users to hard-delete runs uploaded to a Scenario Explorer instance - the current practice is that the modelers have to send me an email and I decide if the deletion is ok (e.g. wrong scenario name -> ok; some error in some timeseries data -> just upload a new version).