Add "Configure Sourcegraph" button support for Gitlab

Created by: erzhtor

For GitHub, when the repository doesn't exist in Sourcegraph and also repository is detected as private, we show "Configure Sourcegraph" or "Repository not found" buttons for Cloud and self-hosted URLs, respectively.

However, there is no similar logic/support for GitLab, even though we support repository syncing with it.

Investigate w/ owners if it is okay to add?