Move user_follow to separate file (#1210)

Also add unit tests

models/fixtures/follow.yml

@@ -0,0 +1,4 @@
+-
+  id: 1
+  user_id: 4
+  follow_id: 2

models/fixtures/user.yml

@@ -26,6 +26,7 @@
   avatar_email: user2@example.com
   num_repos: 2
   num_stars: 2
+  num_followers: 1
 
 -
   id: 3
@@ -56,6 +57,7 @@
   avatar: avatar4
   avatar_email: user4@example.com
   num_repos: 0
+  num_following: 1
 
 -
   id: 5
@@ -72,6 +74,7 @@
   num_repos: 1
   allow_create_organization: false
   is_active: true
+  num_following: 0
 
 -
   id: 6