access inbox management

[elf-pavlik]

#66 doesn't have full handling of access inbox

TODO

• push notification should include interop:grantedBy
• once agent registration is created, remove the receipt from the inbox
• auto-delete receipts from all agents that already have social agent registration
• only allow one access receipt per agent in the inbox, if another one arrived delete it silently
• advanced: detect spam and blacklist sender in ACR

/cc @justinwb

[elf-pavlik]

There could be also an option to remove access inbox all together from the spec: solid/data-interoperability-panel#280