Support GitLab CE/EE

[ffes]

At the office we use a on-premise GitLab CE. It would be great if the extension could support this.

I have added this gitHubDomain setting to my workspace settings.json, but that doesn't help:

"openInGitHub.gitHubDomain": "https://gitlab.domain.local"

Obviously I put in the proper domain name, that matches the git remote. I tried with and without https. I keep getting the warning Unknown Git provider.

On-premise GitLab has exactly the same structure as GitLab.com, just the domain name is different.

[mat-m]

I think the best way is to add a GitLabDomain setting to identify the custom URL as a GitLab one and not a GitHub one.