Hover the feature card only for installable plugins

The hover on the plugins' feature cards should generate a transition and
shadow only for those plugins that aren't already installed. The
animation and raising of the card should have a visual experience of
inviting the admin to experiment with the plugin and install it.

Before this change, all the feature cards were animated, and the
hovering had no real meaning beyond its visual effect.

Change-Id: I813560163bb869640d6c523ddec1bf6a3cccdfb4