fix(ocm): OCM Specification Compliance

[dragonchaser]

supersedes #383

Fixed opencloud-eu/opencloud#1671

Checklist:

• Sharing to/from OpenCloud [patched] works
• Sharing to/from oCIS [patched] works
• Sharing to/from CERNBox works
• Consistent invite and share payloads
• Make sure incoming federated user uuid doesn't collide with internal users
• Backward compatibility?

Based on the:

• generate a dedicated uuid for federated users instead of using the one from the remote instance owncloud/ocis#9927
• ocm: fix federated userid cs3org/reva#4833
• fix(ocm): Fix IDP value in federated user ids cs3org/reva#4933

The issue I raised has happened.

The combined goal of those 2 PRs is:

1. Avoid collision from remote users
2. Preserve the local user IdP info when federating by constructing the opaque_id = user@idp, which is completely fine based on OCM spec, even without the base64

The issue:

• OpenCloud should only base64 encode INCOMING user contact information, not OUTGOING (changed in 4933)

P.S. Tests are probably still outdated or wrong

[micbar]

@dragonchaser Please always add a Type label for the changelog.