Make "online help" link dynamic

Steps to reproduce:

In Studio

• Set Target Platform to 10.1
• Create a new Automation Chain.
• Add the Services > Audit.LogEvent operation.
• Click on the Expression Editor icon beside the Event field.
• Wonder what kind of expression is required in this field.
• Click on the online help link.
  => A tab opens with URL: http://explorer.nuxeo.com/nuxeo/site/distribution/Nuxeo%20Platform-5.6/viewOperation/Audit.LogEvent
  => The following error appears on the page:

  {"entity-type":"exception","status":500,"message":"javax.ws.rs.WebApplicationException: com.sun.jersey.api.MessageException: A message body writer for Java class org.nuxeo.ecm.core.api.NuxeoException, and Java type class org.nuxeo.ecm.core.api.NuxeoException, and MIME media type text/html was not found.\n"}
  

Expected behavior:

• The link should lead to the operation's page in the 10.1 distribution, not 5.6.

Unfortunately, this will also fail because 10.1 is not available in Explorer (see RELEASE-541). In that case:

Expected behavior if target platform is not available in Explorer:

• The link should lead to the operation's page in the most recent available distribution on explorer.