This is a follow up to the gadget regression in T253912 to prevent similar regressions happening again.
The addPortletLink method in core makes the assumption that skins use a class "emptyPortlet" class for hidden classes. This is currently [[ https://codesearch.wmflabs.org/skins/?q=emptyPortlet&i=nope&files=&repos= | utilised only by Timeless and Vector. ]] It shouldn't assume this and it shouldn't assume that it's allowed to turn on hidden portlets - that decision should rest with the skins themselves.
# Acceptance criteria
[] Core methods are added for showing and hiding a portal
[] emptyPortlet class is no longer shipped by Vector
[] vector uses the new mw.util portlet methods to hide/show portals
# Proposed solution
Methods will be added for hiding and showing the portal and the associated class and styles will live in core.
# See also
* {T25515}