Initial version of /health_check endpoint, with tests.

This should resolve mozilla/mofo_system_configurations#70. For folks who can't see the bug, the requirements are as follows:

If `/health_check?elb=true`:
* Do not test database, caches, resources...this is only "Is there a web server here"
* Require no auth, since this will be used by ELB
* Return a 200, no output necessary

If `/health_check` with no params:
* Require auth, or give a 403 (or 401).
* This can be a more vigorous check, of things like resources, services, db, cache servers, etc.
* Ideally return json or text with status of each of those, and @jdotpz can hang health checks off them all and report on specifics with downtime, what caused it, when, timelines, etc.

The pull request is based on https://gist.github.com/dryan/40a4c226d10ba56a1dbe.

The auth required at `/health_check` is HTTP Basic Auth with username **health_check** and password defined by `settings.HEALTH_CHECK_PASSWORD`. If that setting isn't defined, the server will return 501 Not Implemented. 

Right now we don't actually report much--just the server's timezone and whether the database appears to be online.