Limit the number of people that can be followed from one account (#8807)

Configurable soft limit of 7,500, and above that, configurable
ratio of 1.1 * followers, controlled by:

- MAX_FOLLOWS_THRESHOLD
- MAX_FOLLOWS_RATIO

Fix #2311

spec/models/follow_spec.rb

@@ -23,6 +23,20 @@ RSpec.describe Follow, type: :model do
       follow.valid?
       expect(follow).to model_have_error_on_field(:target_account)
     end
+
+    it 'is invalid if account already follows too many people' do
+      alice.update(following_count: FollowLimitValidator::LIMIT)
+
+      expect(subject).to_not be_valid
+      expect(subject).to model_have_error_on_field(:base)
+    end
+
+    it 'is valid if account is only on the brink of following too many people' do
+      alice.update(following_count: FollowLimitValidator::LIMIT - 1)
+
+      expect(subject).to be_valid
+      expect(subject).to_not model_have_error_on_field(:base)
+    end
   end
 
   describe 'recent' do