Set batcache's debug mode to false on production envs

Batcache adds some HTML comments to the end of the output with debug information like page generation time. In some cases this can cause problems with search engines checking content modification dates set in the JSON+LD schema output but finding the page can have a different checksum even if the modified date is the same.

Acceptance criteria:

- [ ] Batcache's debug property is set to `false` by default on production environments
- [ ] There is a config option for controlling the debug mode of batcache
- [ ] The config option is documented