Default follows for new users (#4871)

When a new user confirms their e-mail, bootstrap their home timeline
by automatically following a set of accounts. By default, all local
admin accounts (that are unlocked). Can be customized by new admin
setting (comma-separated usernames, local and unlocked only)

spec/services/bootstrap_timeline_service_spec.rb

@@ -0,0 +1,37 @@
+require 'rails_helper'
+
+RSpec.describe BootstrapTimelineService do
+  subject { described_class.new }
+
+  describe '#call' do
+    let(:source_account) { Fabricate(:account) }
+
+    context 'when setting is empty' do
+      let!(:admin) { Fabricate(:user, admin: true) }
+
+      before do
+        Setting.bootstrap_timeline_accounts = nil
+        subject.call(source_account)
+      end
+
+      it 'follows admin accounts from account' do
+        expect(source_account.following?(admin.account)).to be true
+      end
+    end
+
+    context 'when setting is set' do
+      let!(:alice) { Fabricate(:account, username: 'alice') }
+      let!(:bob)   { Fabricate(:account, username: 'bob') }
+
+      before do
+        Setting.bootstrap_timeline_accounts = 'alice, bob'
+        subject.call(source_account)
+      end
+
+      it 'follows found accounts from account' do
+        expect(source_account.following?(alice)).to be true
+        expect(source_account.following?(bob)).to be true
+      end
+    end
+  end
+end