Default behaviour for making public sites indexable

[LisaFC]

So via #611 I have discovered that checking if HUGO_ENV is set to "production" is not the same as checking isProduction, because Hugo is determined to confuse me. :)

At the moment because we check the former but not the latter, by default our sites are not indexed - you need to set an environment variable in eg your Netlify deployment settings. If we switched to checking isProduction, sites are publicly indexed by default. This would be a (fairly trivial) breaking change.

Thoughts?