Port the build pipeline and test harness to ES. Fix bug 1030309.

Yet to come: request-time dir listing rendering. Right now, you can't see folder listings, but files are fine.
Here's where I stopped, in case you're curious: https://github.com/erikrose/dxr/commit/ebc7d4a6a56887ee47644098b6eafaa57762106b. Anything more on this PR, or shall I merge?