Add A Distinction Between Followers And Friends

Especially with the introduction of chat, I think there needs to be a distinction between followers and friends. I may follow people who have interesting blogs, for instance, but I may only want to set a specific subset of those followees as friends with these privileges:
- Ability to send chat messages / share files
- Ability to see full profile

No files, tests, or entry points are mentioned. Start by clarifying how followers and friends should differ and which existing chat, file-sharing, and profile-visibility behavior is affected. Done means the distinction and its three listed privileges are implemented consistently.

Written by the indexing model from the issue text.