[web] Share with federated users

[tbsbdr]

see @JammingBen comment

[JammingBen]

The basic feature has already been implemented via https://github.com/owncloud/web/pull/9896. However, the whole thing is not yet productized and is missing a few things for that to be possible:

• [ ] finish server implementation on our side (there are still some bugs)
• [ ] add dev docs on the app and how to run the setup
• [ ] Add ocm.openRemotely config to oCIS
• [x] Add ocm.openRemotely config to our dev docs
• [ ] Redesign the ocm web app @tbsbdr
• [ ] Add tests (unit and e2e)
• [ ] Changelog item

[tbsbdr]

obsolete in meantime