Friend behavior

[wmurphyrd]

The goal is to create a friending flow that works like this:

1. Send a friend request
2. Other user must manually approve request
3. Then you become reciprocal friends (i.e. you both follow each other)

Requires immers-space/immers#22

Changes on the client side:

1. Upgrade immers-follow-button to show pending request state and allow user to accept
2. Ability to generate Accept activity
3. Differentiate current friends v. pending requests v. strangers from friends status updates

This could be even better with some notification of request and controls in the react ui to send & accept requests, but I think we've got a MVP here