Some modules failed to load on build. This should be removed once
vue-cli is updated to v4, as it will default to core-js@3.

The current production deployment of the lohrothek-api project is picky
when it comes to being a PROXY_TARGET and because the production
deployment of thekno will use an external origin anyway it’s a good idea
to support it during development.

Setting VUE_APP_APIORIGIN as an environment variable allows users to set
the origin of an external API that should be used for data retrieval.

This path is handled by the lohrothek-api django app.

This adds limited support for environment configuration. Available
options are outlined in the `Custom Enviroments` section in the readme.

For now the only configurable setting is the `apiOrigin` making it
possible to switch the origin of the thekno API without creating a
separate build.

Environment configuration is provided by the `/thekno-env.json`
endpoint.

This is a result of a discussion in
lohrothek-api#75 (comment 4383).

The Feature-Policy header allows us to specify which APIs should be
available in the browser. By reducing the number of APIs and features we
can reduce the attack-surface for any kind of code on the site (handy in
case of an XSS bug) and potentially get some more speed.

see: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Feature-Policy

* enforce lf EOLs
* charset should always be utf-8
* apply to all files

required packages don’t change very often, but package(-lock)?.json
does. so don’t copy before installing the required packages.

node:alpine seems a little unstable for our purposes, so we stay with
LTS releases to be sure.

Urls that don’t contain a filename require an assigned format
in order for Howler to know how to plain them. Deduce this information
from the container and codec properties available on each source.

see https://vue-loader.vuejs.org/options.html#transformasseturls

* now implements aria-role for sliders instead of readonly progress
* add keyboard and knob-drag seek support