Handle multiple versions/years for questions/sections/assignments

[struan]

Probably means we need an overall object to represent a year/marking session and have everything associated with that.

• [x] add model for marking sessions
• [x] update existing models to link to this
• [x] create marking session for 2023 scorecards and use this for existing details
• [x] add table to store current marking session (possibly other details too like current stage?)
• [x] update front end code to load current marking session questions etc
• [x] update front end to add marking session switcher for admins
• [x] add 2025 marking session

[struan]

have cut off date for council as part of marking session, duplicate everything else per marking session - assignments, sections, questions etc - should all fall out by linking section to a session

[struan]

Fixed in #150