Render JS plugins without href on plugin list view

Plugins are linkified on admin plugin list view. If activated, the
Documentation for the plugin is shown. Because JS plugins can not have
Documentation or other resources that can be activated, render them
without href.  To have this information on the client side, extend
REST API with index_url attribute.

Change-Id: I49cab14cc2b01bc4e8cd0c906f4b3bdde1da905c