Remove fix extensions button from extensions dashboard

This partially reverts commit d7d6f54c79a30abfdff36ed1db0931cf002b356a
by removing the UI from the extensions dashboard but leaving
fix_extensions method in instructor.views.tools. This is ultimately a
hack to fix a deeper platform issue so no need to give it an official UI
within the extensions dashboard. Removing in favor of an XSift that runs
the reapply_extensions management command.
catch ValueError in reapply_extensions command

Raised when the course name is not of form ORG/COURSE/NAME
s/fix_missing_extensions/reapply_all_extensions/g
add --dry-run option to reapply_extensions command