denormalize page-list users

[nelsonpecora]

Problem

Currently, the users in each page in the page list include all of their data, not just an ID to look them up. This means that any changes made to the user data will not reflect on old pages, and makes the search mappings brittle.

Solution

Instead, we should store an array of { id, updateTime }, where each id is the <base64:username@provider> of the user. This would allow Kiln to always display the most up-to-date user information, and would remove some of the sanitization code around updating the page list.