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
