Add preview type to hovercards schema. Preview type may either be:
- Regular ("Extract") hovercard - hovercard displays preview of the article in question
- Generic hovercard - hovercard displays the "there is no preview available" card, currently set for articles with no lead section.
- The previewType property is added to the Schema:Popups schema.
- The previewType property is only logged as part of the dismissed and opened actions (ideally but optionally also as part of the tapped settings cog action).
- N.B. that the "generic" preview doesn't have the cog button.