Tune document details in Legalpad

Summary:
Fixes T9245. These picked up some possibly-confusing metadata, like in the screenshot on T9245 where "Subscribers" appears in the middle of the page for no obvious reason.

• Make these pages a little cleaner by removing elements which aren't important for signing agreements.
• Use the last time the actual document text was updated as the modification time, not the last time the "Document" object was modified. The latter will change for trivial things like altering the view/edit policy, but that could be confusing if you see that a TOS was "last updated yesterday" but can't figure out what actually changed (since nothing changed).

Test Plan: Viewed signature page for a document.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9245

Differential Revision: https://secure.phabricator.com/D13982