Lists (#5703)

* Add structure for lists

* Add list timeline streaming API

* Add list APIs, bind list-account relation to follow relation

* Add API for adding/removing accounts from lists

* Add pagination to lists API

* Add pagination to list accounts API

* Adjust scopes for new APIs

- Creating and modifying lists merely requires "write" scope
- Fetching information about lists merely requires "read" scope

* Add test for wrong user context on list timeline

* Clean up tests

app/models/list_account.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+# == Schema Information
+#
+# Table name: list_accounts
+#
+#  id         :integer          not null, primary key
+#  list_id    :integer          not null
+#  account_id :integer          not null
+#  follow_id  :integer          not null
+#
+
+class ListAccount < ApplicationRecord
+  belongs_to :list, required: true
+  belongs_to :account, required: true
+  belongs_to :follow, required: true
+
+  before_validation :set_follow
+
+  private
+
+  def set_follow
+    self.follow = Follow.find_by(account_id: list.account_id, target_account_id: account.id)
+  end
+end