Minify and concatenate client js's

Implements https://github.com/jquery/download.jqueryui.com/issues/67

Minification gains:
- download: 17% (314.6KB -> 261.7KB);
- themeroller: 15% (305.2KB -> 259.8KB);

Highlights:
 1. Grunt task `build-app` runs (a) handlebars, which updates frontend templates, and (b) uglify.
 1. `prepare` runs `build-app`.
 1. On development, download and themeroller indexes still load each .js individually, better for development. On production (see env option on main.js), it uses the minified bundle.
 1. On wp deploy: now, we only expose the bundle minified file. Previously, we exposed every single JS.