Follow -> Accept flow

[sij411]

Description

Handle incoming Follow by deciding to accept and emitting actions.

Proposal

• On Input::ReceivedFollow, store follower (mocked) and emit Accept
• Emit Action::SendActivity(Accept, ...)

Done when

• Unit test: Follow in => StoreFollower + SendActivity(Accept)
• Core does not perform HTTP/storage

Out of scope

• HTTP Signatures verification
• Actor fetching / moderation policy