Module planner drag and drop ordering

Is your feature request related to a problem? Please describe.

I'm not sure which modules I can get through bidding, so I often draft up a few versions of potential module plans. A common pattern is, if I don't get module X I'll try to bid for module Y instead. To visualise this in the module planner, I currently set the colours for module X and Y to be the same, then show/hide them alternatively.

It would be easier for me to visualise these 'substitution groups' if I could get the modules to be adjacent to one another on the list of modules. However, this list is currently limited to three predefined sort orders (module code, exam date, module credits).

Describe the solution you'd like

Drag-and-drop custom ordering for modules in the module list.

Describe alternatives you've considered

Not critical issue, but nice to have. Depending on if and what frameworks were used to generate the module list, it may be a good first issue.

Additional context

None, and thanks for the hard work.

Start by locating the module planner's module list and the code implementing its module-code, exam-date, and module-credit sort orders. Trace how the list is rendered and how planner state is stored, then define custom drag-and-drop ordering that keeps substitution modules adjacent and verify the three existing sort options still work.