Introduce StatusThreadingConcern (#3490)

* Add a StatusFilter class to identify visibility of statuses by accounts

* Extract StatusThreadingConcern from Status

* Clarify purpose of checking for nil account

spec/lib/status_filter_spec.rb

@@ -0,0 +1,65 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe StatusFilter do
+  describe '#filtered?' do
+    let(:status) { Fabricate(:status) }
+
+    context 'without an account' do
+      subject { described_class.new(status, nil) }
+
+      it { is_expected.not_to be_filtered }
+    end
+
+    context 'with real account' do
+      let(:account) { Fabricate(:account) }
+      subject { described_class.new(status, account) }
+
+      context 'when there are no connections' do
+        it { is_expected.not_to be_filtered }
+      end
+
+      context 'when status account is blocked' do
+        before do
+          Fabricate(:block, account: account, target_account: status.account)
+        end
+
+        it { is_expected.to be_filtered }
+      end
+
+      context 'when status account domain is blocked' do
+        before do
+          status.account.update(domain: 'example.com')
+          Fabricate(:account_domain_block, account: account, domain: status.account_domain)
+        end
+
+        it { is_expected.to be_filtered }
+      end
+
+      context 'when status account is muted' do
+        before do
+          Fabricate(:mute, account: account, target_account: status.account)
+        end
+
+        it { is_expected.to be_filtered }
+      end
+
+      context 'when status account is silenced' do
+        before do
+          status.account.update(silenced: true)
+        end
+
+        it { is_expected.to be_filtered }
+      end
+
+      context 'when status policy does not allow show' do
+        before do
+          expect_any_instance_of(StatusPolicy).to receive(:show?).and_return(false)
+        end
+
+        it { is_expected.to be_filtered }
+      end
+    end
+  end
+end