preserve submission timestamp when updating assessment lock state

[davidsbailey]

In response to https://codeorg.zendesk.com/agent/tickets/208820 . Ideally we'd add a submitted_at column to UserLevel instead of relying on updated_at, but that table is huuuge and this fix is straight forward.

The code change in this PR affects the codepath for updating the lock state. For additional context, this data gets to the assessments tab in the client via this call stack:

• code-dot-org/dashboard/app/controllers/api/v1/assessments_controller.rb

  Lines 75 to 76 in 2807be2

  	# GET '/dashboardapi/assessments/section_responses'
  	def section_responses

• code-dot-org/dashboard/app/controllers/api/v1/assessments_controller.rb

  Line 92 in 2807be2

  last_attempt = student.last_attempt_for_any(script_level.levels)

• code-dot-org/dashboard/app/models/user.rb

  Lines 1020 to 1030 in 2807be2

  	def last_attempt_for_any(levels, script_id: nil)
  	level_ids = levels.pluck(:id)
  	conditions = {
  	user_id: id,
  	level_id: level_ids
  	}
  	conditions[:script_id] = script_id unless script_id.nil?
  	UserLevel.where(conditions).
  	order('updated_at DESC').
  	first
  	end

[davidsbailey]

@aoby , could I please have a sanity check from a rails perspective that assign_attributes + save! is equivalent to update!? That substitution is intended to be a pure refactor (no behavior change), and the touch: false param is supposed to be the part that implements the behavior change.