Some tests where comparing actual with expected paths, without taking
into account that paths will be different on Windows.
This commit uses `path.resolve()` to convert expected paths to their
OS-specific form.
PR Close#25671
Previously, Travis pushed the build artitfacts to the preview server.
This required us to use JWT to secure the POST request from Travis, to
ensure we couldn't receive malicious builds.
JWT has been deprecated and we are moving our builds to CircleCI.
This commit rewrites the TypeScript part of the preview server that
handles converting build artifact into hosted previews of the docs.
PR Close#25554
