Don't Allow Users to Follow Private Users

## Problem

Currently, users are able to follow other users who have marked themselves as private. As this isn't particularly private, this provides an unexpected user experience for private users. We should update this logic so that users cannot follow private users.

## Success Criteria
- [ ] Don't allow users to follow other users who are marked as private
- [ ] This should include both removing the button from the front-end and preventing the follow in the `followUser` cloud function backend
- [ ] Notably, we should still allow users to "Unfollow" users regardless of their private status
- [ ] e.g. user A follows User B, then User B goes private. Even though users can't follow User B now, the existing users should still be able to Unfollow (to avoid clogging up their newsfeed
## Intentional Exclusions

This work does *NOT* require us to change anything about processing notifications for users who are already subscribed to another user before the followed user goes private. We still have some product work to do there to explore what we want to happen in that case.