RFE: make invitation workflow symmetric

[glpatcern]

The current OCM invite/accept workflow to create shares between user A and B is not symmetric, in the sense that once A invited B and B accepted, A can share with B but B is requested to invite A to be able to share to A.

This RFE is to evolve the workflow - currently still to be merged from #54 - such that A can choose a two-way invitation (and implicitly accepts the back invitation from B). As discussed with @michielbdejong this requires to create a "back token" for the back invitation process to take place.

To be further detailed with the exact expected sequence of operations.