[entity] Stop linking to external site for external entities This behaviour was ok when we had "true" multi-sources but its rather painful with datafeed sources. Also, it makes absolute_url() costlier than what it should. Besides, it relies on cw_metainformation()['source'] that is pending for removal. Instead, add a link to the original object in the metadata view (the one that displays eid and source at the bottom right corner of the primary view). Related to #15538288.

../../doc/book usr/share/doc/cubicweb-documentation
../../doc/_build/html usr/share/doc/cubicweb-documentation