set up webpack and create bundled wega.js file

[peterstadler]

So, this escalated somehow while I was simply trying to replace the outdated google-code-prettify with highlight.js for #597.

Background

We're are using very old-fashioned Javascript imports by loading each dependency via a dedicated script tag in our page template. That has several drawbacks, e.g., we have to maintain a dedicated WeGA-Mirador repo because there's no way to use Mirador plugins without importing and bundling them. For Highlight.js I couldn't find a simple way to create/get a distributable from the node modules, hence I thought it was high time to tackle the issue of modernizing our Javascript imports in general.

Changes in detail:

1. add webpack as dev-dependency to package.json and add a dedicated ant target "webpack" that invokes the bundling of Javascript files via webpack in our regular build process
2. remove google-code-prettify and add highlight.js from/to page template and package.json; adjust javascript in init.js
3. import Moment.js via the new mechanism (as a proof of concept)
4. add an "API" link to the footer. I needed this for providing a stable reference to $api-base. In fact, the POST request for pushing the XML line-wrap switch to the session was only working for documents but not e.g. persons. See screenshot and notice the "undefined" in the POST request URL after toggling the "Zeilenumbruch" switch:

5. Update the Swagger-UI page footer with the recent changes to the default page template