Simplify dev server

[acelaya]

This PR simplifies the script that runs the development express server. The changes are:

• Remove mustache entirely. The only template we use is actually static html, so we don't need a templating system.
• Expose the images folder as a static source, so that we can access source icon images.

Those two changes make the dev server closer to how the patterns docker container behaves.

With these changes, the templates folder might not make a lot of sense anymore. Maybe we can later consider having a public folder containing the index.html file and the images folder on it. So basically everything which is already static, does not need any building to be served, and we want to be accessible via HTTP.

public
├── images
│   └── icons
│       └── ...
└── index.html

Testing steps

1. Check out this branch
2. Run dev server: make dev
3. Make sure the overall navigation works in http://localhost:4001
4. Verify static images can also be accessed locally http://localhost:4001/images/icons/annotate.svg (any ther icon should also work).

[acelaya]

From express docs:

How do I render plain HTML?
You don’t! There’s no need to “render” HTML with the res.render() function. If you have a specific file, use the res.sendFile() function

https://expressjs.com/en/starter/faq.html#how-do-i-render-plain-html

[lyzadanger]

Thanks! I do agree that we can do further work to simplify the structure of the static assets.

Note that the images/icons directory is referenced by the generate-icons script and will need to be updated if those icons are moved.