Fix error in `PhabricatorSetupIssueView`

Summary:
- Move `prettyPrintJSON()` and make it static.
- Use it from `PhabricatorSetupIssueView`
- Update other `config/` places that use it to call it from the new class.

This fixes a bug in `PhabricatorSetupIssueView` which showed up if the value
was an array and couldn't be rendered by `phutil_escape_html()`.

Test Plan:
- Rendered some config options.
- Went to /config/issue/config.unknown.phame.skins/ without error.

Reviewers: epriestley, btrahan, chad

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2255

Differential Revision: https://secure.phabricator.com/D4411