Following feature

## API

* GET /api/following/:userId/followers
* GET /api/following/:userId/following
* POST /api/following/:userId/follow
* POST /api/following/:userId/unfollow

## Search

* /api/search/followers/:userId
* /api/search/following/:userId
* Piggy-backed on resource_members and resource_memberships to make the following/followers associations. Still on the fence about it but it seems simple enough for now

## Activity

* New activity: following-follow
* New associations: The following activities will route to the followers:
  * Actor's followers will get the activity when they create content, group, discussion
  * If the user becomes a member of a content item, group, discussion, followers get activity

## Other

* Activity Refactored. Introduced "associations" which lets the routing spec become configuration rather than an implementation
* Routing spec moves up to activity level while producing, transforming, and propagation stays at the entity level
* Improved unit tests in some areas to cover new privacy issues that "following" unlocks that was awkward to achieve before
* Centralized visibility and joinable "constants"