separate heartbeat and selftest endpoints, and update specs.

* don't require api key for heartbeat, nor expose tracebacks.
* add tests for heartbeat.
* fix test with hard tabs.

JIRA: FOR-96
Merge pull request #94 from edx/jsa/heartbeat-no-key

don't require api key for heartbeat.
Return 404 instead of 400 when thread not found

Co-authored-by: Greg Price <gprice@edx.org>
Merge pull request #89 from edx/jsa/thread-404

Return 404 instead of 400 when thread not found