Allow issuers to award badges with evidence attached

This is an attempt to resolve #122.

We're doing this by associating uploaded evidence with the [reserved claim codes](https://github.com/mozilla/openbadger/pull/185) that are created when badges are issued from OpenBadger.

When the API client (e.g. CSOL-site) is notified of a new reserved claim code via webhook, it will also check to see if there's any evidence items attached; if so, it will be responsible for downloading all the evidence from OpenBadger before redeeming the code. We need to do all this because the API client is the only entity that knows about evidence, manipulates it as needed (via e.g. video transcoding or image resizing), and presents it in an intelligible way to users.

Things that need to be done in OpenBadger:
- [x] Add a temporary evidence schema that hangs off the claim code schema.
- [x] Add methods to put/get/delete temporary evidence.
- [x] Add documentation for AWS-related environment variables to `README.md`.
- [x] Add a health check for AWS-related functionality.
- [x] Add metadata about how many evidence items a reserved claim code has to relevant API endpoint(s).
- [x] Add an authenticated API route that allows a single evidence item to be streamed to the client with the appropriate MIME type.
- [x] Ensure that redeeming or deleting a claim code also destroys its temporary evidence.
- [x] Document the new API routes in `docs/api.md`.
- [x] Add a view (or modify an existing one) that allows temporary evidence to be uploaded as files, and add/modify the corresponding route.