Added /api/submissions endpoint, allowing listings by learner.

Right now I'm assuming that clients will ask for submission status on a
per-learner basis, e.g. whenever a learner logs in. If we need something
else, we can add additional search criteria later.

Fixes #21.
Added 'mediaType' field to submission evidence schema.

Currently there are only two media types, 'image' and 'link', with
the default being 'link'. The submission detail template simply links
out to links, while it presents a thumbnail of images. Adding more
media types isn't hard.

Fixes #27.