Revert "Reverting "Data migration to add mainstream categories for DCMS""

We have the correct mainstream subcategories now.

This reverts commit 0a8d509790e1282e8116fa64a05835485c1c45b2 to reinstate (a corrected) commit 7fe82c9a425e61ee5dbfb7c8ee92f8ad825668b5.

For: https://www.pivotaltracker.com/story/show/45057197