Shows a timeline of all modified settings Fixes T6545
Will show all settings (no pagination, should be not so difficult to add if needed but most installs won't have hundreds of settings changes)
I'm not happy by how the PhabricatorConfigTransaction object is instructed to render the config keys but i don't see any other reasonable way.
We could always show the keys though.
Test Plan: Changed settings and called the history page
Reviewers: Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Subscribers: Korvin, epriestley
Maniphest Tasks: T6545
Differential Revision: https://secure.phabricator.com/D11088