spec: speed up content migrations specs

1. add some :once's to reduce total runtime from 4:50 -> 3:50
2. split it out into multiple files so that it can be parallelized
   (slowest one is course_copy_quizzes_spec.rb @ 0:50)

Change-Id: I5fd70d9d05f622180fa000e25e982e821ff36d82
Reviewed-on: https://gerrit.instructure.com/38605
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Landon Wilkins <lwilkins@instructure.com>
QA-Review: Jon Jensen <jon@instructure.com>
Tested-by: Jon Jensen <jon@instructure.com>
Product-Review: Jon Jensen <jon@instructure.com>