Hygiene: Tidy up "core" acceptance tests
- Rename the somewhat awkwardly named "popups_core" Cucumber feature and its accompanying step definitions to "previews".
- Remove as many references to Popups and HoverCards as possible.
- Use language consistent with the QUnit unit and integration tests and documentation, e.g. the user dwells on a link.
- Remove settings-related step definitions from the previews step definitions.