Make Maniphest detail page react to viewer capabilities

Summary:
Ref T603. Disable things the user can't use, allow logged-out users to get a reasonable version of the page.

Also allow logged-out users to view edit history of comments if they're able to see the object.

Test Plan: Viewed Maniphest detail as a logged-out user, got a largely sensible page.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7124