Port MySQL settings to PHP

Summary:
  - Ports MySQL settings to PHP.
  - Removes "mysql.retries" -- this existed only because Magic Numbers Are Bad, but there is no concievable reason it should ever be set to anything other than 3.
  - Introduced "Hidden" config, which isn't visible from the web (for SaaS, we'll just mark anything with secret keys as "hidden").
  - Introduced "Masked" config, which will be masked in darkconsole once that gets updated.
  - "Hidden" implies "Masked" and "Locked".
  - Moved "storage.default-namespace" here -- it probably makes more sense than core; this was my bad in T2255.
  - Put cancel button back for hidden/locked config.
  - Introduce 'class' config type.

Test Plan: Viewed MySQL options. None are editable.

Reviewers: codeblock, btrahan

Reviewed By: codeblock

CC: aran

Maniphest Tasks: T2255

Differential Revision: https://secure.phabricator.com/D4326