Since our initial launch of LSST the Docs in 2016, Fastly has incorporated built-in configuration options for several of the things that we originally needed to add custom configuration for. There are also configuration aspects that are no longer relevant.

This ticket is to make these configuration changes. Note that DM-24955 is where we're fixing directly index support with query strings.

1. Switch `Redirect to HTTPS` request setting to the built-in `Force TLS and enable HSTS`. Also, drop the "If not TLS" condition to "Reset original host for HTTPS redirect" https://docs.fastly.com/en/guides/enabling-hsts-through-fastly
2. Drop the RTD redirect Content header settings.
3. Stop using `900` range for internal states. Instead, use `600`. Based on Howell's blog post from McDowell.
4. Change headers like http.location to `http.Location`.
5. Fastly's built-in error page support (e.g. 404).
6. Fastly's built-in gzip support