[Feature] Customize base edit_uri

[Kevin]

A way to use the edit_uri even when the `--force-edit-url' flag is set would be very helpful.

Our project is currently hosted on GitLab and the GitHub is only a mirror. So we'd like to link the edit_uri to our GitLab page. Unfortunately, the edit_uri in each manifest is automatically rewritten to the GitHub URI when the --force-edit-url flag is set.

https://github.com/drasyl-overlay/drasyl/blob/master/docs/mkdocs.yml#L10