attachment gc improvements + support ContentMigration
attachment gc improvements + support ContentMigration

fixes CORE-58

test plan:
- successfully deletes old ContentMigrations, including those tied to
old ContentExports

Change-Id: Ib9ca545da8a6929f495317bba08d0103acbaf1cc
Reviewed-on: https://gerrit.instructure.com/143330
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Tested-by: Jenkins
Product-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
QA-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>

djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,instructure/canvas-lms