Users need to have their `participant_activities` pre-filled in order for the new routes to work

[seidior]

Details and resources

In order for the new routes (#469) and frontend (#470) to work correctly, we need to pre-fill the participant_activities table with placeholders for the assignments for the active user for each program in the database.

We also need to remove Cohort 5, since its dates are non-final.

This can be accomplished by modifying the existing migration to remove Cohort 5. Then, a new migration can create a stored procedure that we can call from the backend the first time a user loads the calendar. Finally, the backend needs to be adapted to call the stored procedure the first time (and only the first time) the calendar is loaded.

This is not a requirement for this feature to work, and it should be merged after #470 in case it doesn't end up being completed by go-live.

Checklist

• [x] This issue is linked to the appropriate project.
• [x] This issue has appropriate labels.

[phoebehala]

Those are activities we currently have with 'assignment' type.

INSERT INTO principals () VALUES ();
SET @principalId = LAST_INSERT_ID();

INSERT INTO participant_activities (program_id, activity_id, principal_id, completed) VALUES
  (1, 1, @principalId, 1),
  (1, 83, @principalId, 1),
  (1, 88, @principalId, 1),
  (1, 8, @principalId, 0),
  (1, 48, @principalId, 0),
  (1, 92, @principalId, 0);