Migrate local disk storage setup check

Summary:
Migrate to the new hotness. Also:

  - Remove a string test, which is now impossible since the config will repair itself and raise a type error.
  - Restore the header even in /config/ -- this check is kind of hacky and it feels a bit more natural now that it's above the menu.

Test Plan: Set my local disk path to something invalid, verified I got a setup error.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2228

Differential Revision: https://secure.phabricator.com/D4497