ENH: Disable search on extension details page

Tweak handling of the extension search field to disable it when looking
at the detail page of an extension, as it makes no sense in this case.
Also disable it (for now) when managing installed extensions.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23130 3bd1e089-480b-0410-8dfb-8563597acbee