ci(docs-infra): change AIO preview server stuff to pull builds from CircleCI

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.

aio/aio-builds-setup/dockerbuild/scripts-js/lib/common/constants.ts

@@ -1,3 +1,4 @@
 // Constants
+export const AIO_DOWNLOADS_DIR = '/tmp/aio-downloads';
 export const HIDDEN_DIR_PREFIX = 'hidden--';
 export const SHORT_SHA_LEN = 7;