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)

app/services/bootstrap_timeline_service.rb

@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+class BootstrapTimelineService < BaseService
+  def call(source_account)
+    bootstrap_timeline_accounts.each do |target_account|
+      FollowService.new.call(source_account, target_account)
+    end
+  end
+
+  private
+
+  def bootstrap_timeline_accounts
+    return @bootstrap_timeline_accounts if defined?(@bootstrap_timeline_accounts)
+
+    @bootstrap_timeline_accounts = bootstrap_timeline_accounts_usernames.empty? ? admin_accounts : local_unlocked_accounts(bootstrap_timeline_accounts_usernames)
+  end
+
+  def bootstrap_timeline_accounts_usernames
+    @bootstrap_timeline_accounts_usernames ||= (Setting.bootstrap_timeline_accounts || '').split(',').map { |str| str.strip.gsub(/\A@/, '') }.reject(&:blank?)
+  end
+
+  def admin_accounts
+    User.admins
+        .includes(:account)
+        .where(accounts: { locked: false })
+        .map(&:account)
+  end
+
+  def local_unlocked_accounts(usernames)
+    Account.local
+           .where(username: usernames)
+           .where(locked: false)
+  end
+end