[#316] Disable follower lists

Issue: #316 

- [x] Only sysadmins can see the lists of users that are following a user, dataset or group (`*_follower_list` APIs)
- [x] Only sysadmins or the user herself can see the lists of users, datasets or groups that a user is following (the `*_followee_list` APIs)
- [x] `/follower/` pages should 401 if not a sysadmin
- [x] Remove `Follower` tabs on user, group, package pages
- [x] Disable "started following" activities
- [x] Updated failing tests
- [x] Added new tests for the new auths