[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.