R1:96839d35f49c

Detect and raise setup warnings from within Phabricator

Summary:
This is basicaly a light version of D4286. The major problem with D4286 is that it's a huge leap and completely replaces the setup process in one step.

Instead, I want to do this:

- Add the post-setup warnings (yellow bar with "6 unresolved warnings...").
- Copy all setup checks into post-setup warnings (so every check has an old-style check and a new-style check).
- Run that for a little bit and make sure it's stable.
- Implement…