Upload multiple outcomes and run multiple sensitivity analyses

This came up as a request in a meeting yesterday and I also noticed that a lot of published reviews use multiple outcomes. We could allow multiple outcomes to be uploaded and then the one to analyse would be chosen in `setup_configure`. The potential difficulty would probably be in maintaining backwards compatibility for a single outcome.

Start by tracing the current single-outcome upload and configuration flow, especially `setup_configure`, to understand where the selected outcome is stored and used. Compare that flow with the requested multiple-outcome and multiple-sensitivity-analysis behavior. Done means multiple outcomes can be uploaded and selected for analysis while existing single-outcome inputs continue to work.

Written by the indexing model from the issue text.