Allow working with submodules in WebIDE Edit mode

When working with git submodules, you can't update refs in Web(IDE) nor add / delete them.

Please allow WebIDE / edit mode to add / update a native git submodule.

When Updating a Submodule and at least if submodule is hosted on gitlab too, you should provide a graph revision picker

/label ~"submodules"

Edited May 18, 2018 by Marcel