Side by side compare two document versions

[amir-zeldes]

Currently it's possible to open multiple documents in multiple windows, but if an admin wants to compare annotations from different users there's no good way of doing so.

1. Admins should have a way of opening some user's version of a document. The fact that the document is not by the current user should be marked somewhere in the window.
2. We could consider some dedicated 'compare' mode but this might be overkill