Revamp OCM invitation workflow

[gmgigi96]

This is part of a series of PRs for having OCM in production.
In particular, this is regarding the invitation workflow.

Improvements/changes in this PR:

• Refactored OCM Invite manager
• Updated /invite-accepted endpoint to the last OCM APIs spec (Invitation workflow for OCM, take 2 OCM-API#54)
  • Introduced mutual exchange of user information on invitation accept
• Add errors as specified by the OCM and cs3apis specs (Add more errors on invite-accepted endpoint OCM-API#58, Add error codes to InviteAPIs cs3apis#198)
• Allow user to specify a description when generating an invitation token
• Split old ocmd http service in two services (OCM and sciencemesh) -> Closes OCM: required changes to comply with the invitation workflow #3540
  • ocm services contain only the implementation of the OCM APIs (https://cs3org.github.io/OCM-API/docs.html)
  • sciencemesh is meant for reva clients (like web) for interacting with tokens and shares in OCM
• Written integration tests for the invitation workflow (both grpc and http)

[glpatcern]

@michielbdejong from this point on we're on the new OCM endpoints, therefore you can start adapting the ownCloud/Nextcloud sciencemesh apps code according to cs3org/OCM-API#54 and our discussion. I think we could tag a Reva version after we also implement the shares properly and especially after you give your green light that the apps are OK.