Add remove from followers api (#16864)

* Add followed_by? to account_interactions

* Add RemoveFromFollowersService

* Fix AccountBatch to use RemoveFromFollowersService

* Add remove from followers API

spec/services/remove_from_follwers_service_spec.rb

@@ -0,0 +1,38 @@
+require 'rails_helper'
+
+RSpec.describe RemoveFromFollowersService, type: :service do
+  let(:bob) { Fabricate(:account, username: 'bob') }
+
+  subject { RemoveFromFollowersService.new }
+
+  describe 'local' do
+    let(:sender) { Fabricate(:account, username: 'alice') }
+ 
+    before do
+      Follow.create(account: sender, target_account: bob)
+      subject.call(bob, sender)
+    end
+
+    it 'does not create follow relation' do
+      expect(bob.followed_by?(sender)).to be false
+    end
+  end
+
+  describe 'remote ActivityPub' do
+    let(:sender) { Fabricate(:account, username: 'alice', domain: 'example.com', protocol: :activitypub, inbox_url: 'http://example.com/inbox') }
+
+    before do
+      Follow.create(account: sender, target_account: bob)
+      stub_request(:post, sender.inbox_url).to_return(status: 200)
+      subject.call(bob, sender)
+    end
+
+    it 'does not create follow relation' do
+      expect(bob.followed_by?(sender)).to be false
+    end
+
+    it 'sends a reject activity' do
+      expect(a_request(:post, sender.inbox_url)).to have_been_made.once
+    end
+  end
+end