add option to remove dates when importing/copying a course

test plan:
 - notice the date_shift_options[remove_dates] parameter in the
   content migrations API documentation
 - using the content migration api, import a course package
   specifying date_shift_options[remove_dates]=true
   * confirm dates are removed in the copied course, including
     - assignments: due, available, peer-review
     - announcements: post delay
     - quizzes: due, available, show-correct-answers
     - events: start, end
     - modules: unlock-at
 - also try a course copy content migration, and confirm that
   the dates are removed
 - specify both remove_dates and shift_dates, and confirm that
   you get a "bad request"

fixes CNVS-14180
fixes CNVS-14182

Change-Id: Ia07be2883ec4315d4a1e7fbcd61117a4531229d1
Reviewed-on: https://gerrit.instructure.com/38298
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Jon Willesen <jonw@instructure.com>