[console] Add a pure-Javascript console implementation

[mbrukman]

For additional ease of use, it would be nice to have a pure-Javascript version of the console that does not require hosting a Google App Engine app, thus simplifying deployment and configuration even further.

This would still need to be hosted with a web server somewhere, because OAuth2 will request an endpoint, so it will either need to run via a local server, e.g., python -m SimpleHTTPServer 8000 or it can be hosted on a static server, such as Google Cloud Storage or GitHub Pages.

Since Javascript would allow any user to inspect any file content (due to all content being visible to the client-side browser), to authenticate with Google Cloud Platform, rather than embedding our own API key, users would have to provide their own key, which could be stored in Javascript local storage for ease of management.

Ideally, the pure-Javascript console and the current App Engine would be able to share significant fraction of client-side code (HTML, CSS, Javascript) so we don't have to maintain two near-identical trees.