MVP backlinks

Split from #196 (closed) to define a specification of backlinks required for the federation MVP to work

• Inbox for containers and resources
• Integration of DjangoLDP_ActivityPub for notifications
• Sender: send Add activity
• Sender: send Delete activity
• Receiver: Add
• Receiver: Delete

added P1 + 1 deleted label

mentioned in issue management/core-team#3

marked the checklist item Sender: send Add activity as completed

mentioned in merge request djangoldp-circle!20 (merged)

marked the checklist item Receiver: Add as completed

marked the checklist item Sender: send Delete activity as completed

marked the checklist item Receiver: Delete as completed

changed the description

closed

changed milestone to %0.7