Accept OCM to groups

[thepeak99]

Please ensure this PR is merged:

owncloud/core#40589

[michielbdejong]

blocked on cs3org/ocm-test-suite#33

[thepeak99]

There have been new comments, please address.

[navid-shokri]

I commit my changes there:
pondersource/core#17

@thepeak99 please review this PR and merge it into accept-ocm-to-group