Shift (partially) to Webpack (AdvisorySG#85)

The primary purpose of the shift was to enable ES6-style imports in
`main.js`. Most dependencies have been shifted to being managed under NPM which
should make upgrading them much easier.

The following deemed unnecessary dependencies have been removed:
1. MD5

Notes:
1. OWL Carousel has been retained in its original form as there is no direct
   replacement currently. See https://www.npmjs.com/package/owlcarousel to
   verify that the only other dependency aside from the JS library is the CSS
   spreadsheet. The minified JS file has been replaced with the original
   source code for easier debugging.
2. jQuery requires a hack when doing ES6-style imports, see
   https://stackoverflow.com/questions/35358625/jquery-is-not-defined-when-use-es6-import
   for more details.

assets/js/jquery-global.js

@@ -0,0 +1,3 @@
+import jquery from 'jquery';
+window.jQuery = jquery;
+window.$ = jquery;