Add the latest editors name to the task

[ramfox]

Or perhaps the original author. But I'm going with the latest editor for now.

• add UI to get the user's name when they create or join a list
• adjust data model of doc to expect users to add a "name" entry when they join, which allows other users to associate their author id with a name
• add latest_editor field to Todo
• add UI to show the latest editor's name