Use React.js to render the frontend

[JoshRosen]

This pull request rewrites the front-end to use React.js. The various views are now rendered in the browser using JSON sent from the backend, which should significantly improve performance and will make it easier to implement several features that I'm planning.

[nchammas]

Josh, should I hold off on any work to add additional filtering (like "All" or "Stale" tabs) until after this is done?

[JoshRosen]

@nchammas Yeah, I'd hold off for now, since this is going to be a pretty large set of changes. I'll open a new PR to merge in the final JS rewrite patch once we're done.

[JoshRosen]

This branch is now being served on the live site: https://spark-prs.appspot.com/ (I have the old version deployed under a different version number, so it's a one-click rollback if anything goes wrong).

I added a loading screen plus more caching on the backend, so hopefully the page load time is reasonable. Please let me know if you encounter any blocker issues when using the site; if everything looks good, I plan to merge this tomorrow or Tuesday.

@nchammas: this adds the "all" tab that you wanted (as the default tab).

@liancheng: this should fix #29; the "Test with Jenkins" button should now open pages in a new window / tab.

[shivaram]

@JoshRosen Nice work ! The page load times are a bit higher. Am not sure if this is mostly due to caching.

One minor thing I noticed -- The top tabs of module names don't seem to be recognized as links (i.e my mouse cursor doesn't change when I hover over them). They do work fine when clicked though

[JoshRosen]

Since the site seems to be working okay, I'm going to merge this in now to make it easier for people to contribute patches.

@shivaram Good catch w/ the tab mouse cursors; I've opened #35 so we don't forget to fix this.

[nchammas]

Nice work on this Josh!