Some minor change and spec for Account (#3813)

* Introduce domains method to Account relation

Account had followers_domains method, which was excessively specific.
Let relation of Account have domains method instead.

* Move follow_mapping in Account to AccountInteractions

* Introduce shared examples for AccountAvatar inclusion

* Cover Account more

spec/support/examples/models/concerns/account_avatar.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+shared_examples 'AccountAvatar' do |fabricator|
+  describe 'static avatars' do
+    describe 'when GIF' do
+      it 'creates a png static style' do
+        account = Fabricate(fabricator, avatar: attachment_fixture('avatar.gif'))
+        expect(account.avatar_static_url).to_not eq account.avatar_original_url
+      end
+    end
+
+    describe 'when non-GIF' do
+      it 'does not create extra static style' do
+        account = Fabricate(fabricator, avatar: attachment_fixture('attachment.jpg'))
+        expect(account.avatar_static_url).to eq account.avatar_original_url
+      end
+    end
+  end
+end