Glean dictionary "Source" link should link to searchfox for applications in mozilla-central

Right now getting blame for the metrics.yaml file or using other searchfox functionality isn't as intuitive. The Code Search link doesn't list the metrics.yaml definition line, and anyway coming from an m-c world it is kind of surprising that the links don't just go to searchfox anyway.

See also #2245 and #2248 , which also center around broken searchfox links.