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_feed.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+class ListFeed < Feed
+  def initialize(list)
+    @type    = :list
+    @id      = list.id
+  end
+end