Allow editing a remote Swagger file

[marla-singer]

As API manager I want to edit my earlier uploaded swagger file

The initial doc contain info that file is provided as JSON object. But the APInf platform storages files as a link (i.e. http://localhost:30303/gridfs/DocumentFiles/id/123)

TO DO:

• Implement editing a remote Swagger file

Reference: https://github.com/apinf/platform/issues/2780

[tulir]

There are currently two message listeners. The Space auth listener is at space.js#L51-L56 and the Swagger receiver is at app.js#L106-L114. The remote file handling should probably be added to the Swagger receiver, as the functionality is almost the same.