Show "Last Writer" and "Last Write At" in the UI, add more documentation

Summary:
Ref T10751. Make the UI more useful and explain what failure states mean and how to get out of them.

The bin/repository thaw command does not exist yet, I'll write that soon.

Test Plan: {F1238241}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10751

Differential Revision: https://secure.phabricator.com/D15766