Users should be able to accept registrations based on access receipts received

[angelaraya]

Scenario:

Social agent A shared data with social agent B. B does not have a registration for A.

[elf-pavlik]

#45 and #46 provide basics for that, based on notification with webId containing grantedBy from the Access Receipt. Based on that they get navigated to add-social-agent view which shows them this WebID.

I start documenting a possible direction for SAI invitation in https://hackmd.io/@elf-pavlik/rk-uN-R-s
This is a little further down the road but we most likely need it for the users not to have verified WebIDs on their own.