Add a setup issue to warn about an unconfigured upload limit

Summary:
Ref T3354. There's no way for us to test most of the config options which actually affect this limit, so the Phabricator config is basically a canary value to indicate "the administrator hasn't configured anything yet".

Raise a setup issue if it isn't set. There's a trail to get here from Files, but we've de-emphasized the old-school upload form so it's hard to unearth.

Emphasize the warning that you need to read the documentation and configure like 30 other things to make this work.

Test Plan: Cleared my config, verified I got the issue, read it, set my config, issue went away.

Reviewers: jamesr, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T3354

Differential Revision: https://secure.phabricator.com/D6185