Sourcegraph follow-up: fix jump to definition on self hosted instance

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

• @pslaughter started a discussion:

  issue: @christinaforney @felixfbecker Looks like the integrated jump to definition isn't taking the port and protocol into account 😞

  20191113_sourcegraph_bug_integrated_go_to_definition

  I suggest either using the global gon.gitlab_url variable to help build the path, or you can try building a relative path, but you'll need to be aware of gon.relative_url_root.

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  If it seems unfeasible to handle this in this iteration, are we open to reverting back to the "Open in Sourcegraph" functionality?

/cc @felixfbecker @christinaforney