Create a parent class to group Todos

What does this MR do?

This MR is the first step on implementing #24978 (moved), which will group todos on todo events, to help fix the problem where you don't get a new todo if you have an active one for the same event.

The first part I implemented was the data, as having valid data to work with helps a lot when creating the other parts.

Are there points in the code the reviewer needs to double check?

This MR is still under development, so there's still lots of work to do.

As of now, the following is implemented:

• A new model called TodoEvent
• A migration to both create the new todo_events and populate the todo_event_id field on todos. This migration was handwritten to optimize both run (wall) time and reduce the number of locks, so the code is more complex than what's necessary.

There are still no tests, they are going to be added ASAP, and things might break on the migration, so more attention is necessary.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #24978 (moved)