Use external links in Cross References im-table

Some objects like genes have a Cross References im-table which on the legacy webapp will have the cells link externally to specific resources (dependent on configuration from web properties https://github.com/intermine/flymine/blob/867fdac07b99c2cc74b57250fedd67035ef584ee/webapp/src/main/webapp/WEB-INF/web.properties#L388).

We will first need to add a new web service (or reuse an existing one, perhaps /webproperties) to expose the xreflink properties to Bluegenes. Then we'll need to pass that to im-tables-3, so that it knows what URL to point to.