Enable editing of wiki files with GitLab backend

[GordianDziwis]

• [x] I agree to follow the Code of Conduct that this project adheres to.
• [x] I have searched the issue tracker for a feature request that matches the one I want to file, without success.

Is your feature request related to a problem? Please describe. With GitLab as an backend it is not possible to open files associated with the wiki of a repo.

Describe the solution you'd like Being able to edit draw.io flies stored in the project wiki repository.

Additional context When the repository is: [email protected]:foo/bar.git The associated wiki repo is: [email protected]:foo/bar.wiki.git

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