Flexible Grant categories

[estyxx]

What

This PR refactors the grants and reimbursements system, moving from hardcoded monetary fields in the Conference mode to a category-based reimbursement model. It adds new models for GrantReimbursementCategory and GrantReimbursement, migrates existing data, and updates the Django admin interface to support the new structure.

Database and Data Model Refactoring:

• Introduced new models: GrantReimbursementCategory (defines reimbursement types per conference) and GrantReimbursement (records actual granted amounts per category per grant)..
• Migrated existing grants and conferences to the new model, including backfilling reimbursement categories and amounts, and removed the old fields (accommodation_amount, ticket_amount, travel_amount, approved_type, total_amount) from the Grant model and related fields from the Conference model.

See the proposal: https://www.notion.so/pythonit/Grants-3-0-359642da7c094f3d9445fe0e05b7937c

ToDo

Re-introduce the automatic assignment of the grant travel amount based on the country type.