reduce top-margin on content when reviewing
Style tweaks to user profile
tweak padding and alignment for cards being reviewed
Add API for bulk export of cards

This makes it easy to export data for munging by some tool. The next
step is to also allow importing data, good for import from another
system or importing tool-generated cards.

This is littered with TODOs, but hey, it's something.

Test plan:
I visited /api/cards/bulk_export and saw JSON for my cards.