feat(build): Turn on mangling for uglify (#1841)

Turns on mangling for uglify (minified javascript for production).

Previously this might've caused issues with AngularJS, but since we are now using `ngAnnotate`, those issues are gone.

https://github.com/mishoo/UglifyJS2#minify-options

gulpfile.js

@@ -152,7 +152,7 @@ gulp.task('uglify', function () {
   return gulp.src(assets)
     .pipe(plugins.ngAnnotate())
     .pipe(plugins.uglify({
-      mangle: false
+      mangle: true
     }))
     .pipe(plugins.concat('application.min.js'))
     .pipe(plugins.rev())