Repair invalid configuration by setting values back to defaults

Summary:
When configuration is set incorrectly (e.g., of the wrong type), detect and repair it by setting it to the default value. A setup warning will be raised separately.

Notably, this removes the need to hard-code all the class types.

This runs separately from the "invalid config" check because we need to run it on every page, but do setup checks only once per restart (some of them are slow).

Also dirty setup when we edit configuration.

Test Plan: Set config incorrectly on purpose, saw Phabricator correct it on restart and on every subsequent page load until it was fixed.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T2292

Differential Revision: https://secure.phabricator.com/D4492