Provide compatibility for Gadget Spec resources fetching

Gadget Specs resources have moved from :

• /site/gadgets/ {gagdet}/{resourceName}
  to
  - /nxthemes-lib/{resourceName}
  
  We should provide a fall back so that calls to /site/gadgets/{gagdet}

  /

  {resourceName} generate a redirect to the target Web resource in /nuxeo/css/{resourceName}

  or /nuxeo/scripts/

  {resourceName}

The goal it double :

• avoid breaking "old" custom gadgets when migrating to 5.6
• make IDE generated gadget work in 5.6

NB : we must add a deprecation log before doing the redirect