Propagate start_at date during assessment import if other dates absent

[damianhxy]

Your environment
master (5e33614)

Steps To Reproduce
Import an assessment whose .yml file only defines the start_at field.

Current behavior
All fields (due_at, start_at, end_at, visible_at, grading_deadline) are set to the current date + 1 day.

Expected behavior
All fields should take on the value of the start_at field.

Notes
This is to make it more convenient to create a non-submittable assessment (used by 15-122). The relevant logic lies in assessment.rb#deserialize.

Also note that visible_at is an unused field and ought to be removed.