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* app's frontend and backend are combined and served together from Node.js

* add code block to demonstrate build process in a more readable way
  (other than the Makefile)
* emphasise the sensitivity of the JWT token

* NodeJS: v16
* NPM: v8
* Jest: v27
* Mongoose: v6

And fix some small issues along the way

APP_NODE_MODULE_DIRS contains two paths. Quoting that var breaks
the command and the node_module folders were not deleted.

* remove 'renders a todo add' test since it was a) the wrong place
  and b) already implemented by PR #36
* fix indentation and make sure that there is actually a user that
  the test tries to delete

* bump NPM from v6 to v7
* regenerated package lock files
* bump Node from v12 to v14