Sourcegraph follow-up: add external link icon to popover

The following discussion from !16556 (merged) should be addressed:

• @pedroms started a discussion: (+5 comments)

  @pslaughter is it possible to add the external-link icon to the popover links?

  

As mentioned in !16556 (comment 243629228), “that "Go to definition" always jumps to the definition on GitLab (if the target repository is on GitLab)”, so the only external link that required the icon is “Find references”.