add user follow feature

spec/factories/user_follows.rb

@@ -0,0 +1,6 @@
+FactoryBot.define do
+  factory :user_follow do
+    follower { nil }
+    followed { nil }
+  end
+end

spec/factories/users.rb

@@ -1,5 +1,5 @@
 FactoryBot.define do
   factory :user do
-    username { "xpto" }
+    username { Faker::Internet.username.gsub(/[^0-9a-z ]/i, '') }
   end
 end