refs https://github.com/TryGhost/Team/issues/1004
- adds new `{{products}}` helper behind `multipleProducts` flag
- `{{products}}` outputs a string with list of products that have access to specific post when used in a post context in theme
- outputs empty string when used out of a post context and without access to `visibility` property
- uses all available posts for a site via the global products data
- updates {{content}} helper cta to use this new helper to show list of tiers with access to post