Rewrite API

[wengole]

Currently this stores a massive mapping of userids to their unread content UIDs.

This is bad. After speaking with @mattss a much better solution was formulated:

• Upon creation of a "todo", add an todo to all selected users as annotation on the user
• Simultaneously add an "unread" userid to the content as an annotation

These changes would be handled by the API from the same calls.