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.
The previous clean-up code for PR directories on the preview server assumed that
all directories were named after the PR number. With the changes introduced
in #17640 it is possible to have PR directories that do not follow that naming
convention (e.g. "non-public" directories).
This PR ensures that both public and non-public directories are removed when
cleaning up.
