closes https://github.com/TryGhost/Team/issues/466
- upgraded kg-default-cards to include paywall card
- extracted `htmlToPlaintext` from post model to shared util for re-use
- updated post-gating to set html+plaintext to the free preview if a paywall card has been used
- re-generates plaintext from the truncated html using `htmlToPlaintext` util
- display free content in the `{{content}}` helper via the default CTA template
