(stable) Split setup checks into "preflight" and "normal" checks

Summary:
Ref T11589. Currently, initialization order is a bit tangled: we load configuration from the database, then later test if we can connect to the database.

Instead, I'm going to do: preflight checks ("PHP Version OK?", "Extensions installed?"), then configuration, then normal setup checks.

To prepare for this, flag core checks as "preflight" and add a setup panel to visually confirm that I didn't miss anything.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11589

Differential Revision: https://secure.phabricator.com/D16499