According to the docs, the response of GitHub's [PR files API][1]
_"includes a maximum of 300 files"_. This means that if a PR contains
more files, it is possible that not all files are retrieved (which
could, for example, give a false negative for the "significant files
touched" check - not likely but possible).
This commit fixes it by using paginated requests to retrieve all changed
files.
[1]: https://developer.github.com/v3/pulls/#list-pull-requests-files
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.
