Add rpcexplorer to webapp hierarchy.

web/apps/rpcexplorer/gulpfile.js

 /*
   Copyright 2016 The LUCI Authors. All rights reserved.
   Use of this source code is governed under the Apache License, Version 2.0
   that can be found in the LICENSE file.
 */
 
 'use strict';
 
 // Include Gulp & tools we'll use
 var gulp = require('gulp');
 var luci = require('../gulp-common.js');
+var path = require('path');
 
 luci.setup(gulp, {
   dir: __dirname,
+  includes: function(gulp, layout) {
+    return gulp.src([
+        path.join('inc', 'bower_components', 'bootstrap', 'dist', 'css',
+                  'bootstrap.min.css'),
+    ]).pipe(gulp.dest(layout.dist('inc/bower_components/bootstrap/dist/css')));
+  },
 });