Hygiene: Tidy up "core" acceptance tests

Changes:

• 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.

Change-Id: Ibc0d07e8394aeb640f9efaabfe10be5317bf4b8c