Fix Live Connect oauth2 callback in Web UI only

Live connect is not able to authenticate with providers when Jsf UI is not installed.
Error on the callback url.

Reproduce

• Install Web UI and Live Connect (no Jsf UI)
• Fill a provider (eg Box)
• Click on create document
• Go to import tab
• click Box link
• On popup authenticate
  => A error will appear

  {"entity-type":"exception","code":"com.sun.jersey.api.NotFoundException","status":500,"message":"null for uri: http://webui-demo.nuxeo.com/nuxeo/site/oauth2/box/callback?code=BkB4YxrmxVBR27qxFIkCmbmCl4h"}
  

If Jsf UI is install, it does not result in error.