Destroy all related todos when removing a project

Summary

When a a project that has related todos is removed, if we access the todos page with an user that have a todo for that project an internal server error occurs.

Steps to reproduce

1. Create a new project
2. Create a new issue and assignee to an user
3. Remove this new project
4. Login as user
5. Try to access the todos page

Expected behavior

Should destroy all related todos when removing a project.