fix go-to-gitlab link to branch, commit range and line
Created by: vanesa
Similar to https://github.com/sourcegraph/sourcegraph/issues/294, Go-To-Gitlab buttons were not linking correctly to branches, commit ranges and lines.
Checking for externalURL.serviceType === 'gitlab'
and applying the same solution as with GitHub fixes #484 .
This PR updates the CHANGELOG.md file to describe any user-facing changes.