Read configuration from env variables

Related to #747 (closed)

Currently the dbnomics-website application is built with Webpack which stores the configuration variables in the produced JavaScript bundle.

This does not allow running the application from a container image that would pass customized values as environment variables.

Now that the application was ported to Svelte 3 and latest Sapper version (cf dbnomics/dbnomics-website!5 (merged)), it's possible to achieve that.

Tasks

• use Sapper session store to pass environment variables from NodeJS context to the client-side context
• update the Dockerfile to use environment variables instead of build args (ARG directive)
• replace Ansible deployment by a Kubernetes one (cf dbnomics/dbnomics-website!8 (closed)) not yet; work is in progress in #747 (closed)