Added /health_check endpoint, with health check at startup too.

The health check ensures that s3, aestimia, openbadger, and MySQL connections are all functional. Works with both s3 and fake s3.

Currently the cookie secret is used as the password at /health_check but we can change this later in app.js.

Once we figure out what session store we're using in #379, we should also add a session store health check.