Define the 2025 season dates & setup invoicing
This MR adds the 2025 season information to season.py. In order not to break any existing code, the season is added as "invisible", meaning the functions to find it by date or by slug do exist, but it is not exposed in any public page.
This MR also adds the relevant factory for events in 2025.
This MR also defines the fees for 2025.