tmp refactor

Refactor tmp projects yet again.

- Represents tmp projects as `tmpstyle://<path to real project>` and `tmpsource://<path to real project>` rather than bogus `tmstyle:///tmp` + `tmsource:///tmp`
- Updates middleware to be more sane

Cleans up APIs nicely and lays groundwork for actually implementing https://github.com/mapbox/mapbox-studio/issues/507