frontend: narrow down DefinePlugin into EnvironmentPlugin

[wh0]

the previous DefinePlugin

new webpack.DefinePlugin({
    'process.env': JSON.stringify(process.env)
})

caused index.js

elasticsearchMappingSchemaVersion: parseInt(process.env.ELASTICSEARCH_MAPPING_SCHEMA_VERSION),
elasticsearchUrl: process.env.ELASTICSEARCH_URL || '/backend',
elasticsearchUsername : process.env.ELASTICSEARCH_USERNAME || ...,
elasticsearchPassword : process.env.ELASTICSEARCH_PASSWORD || ...,
nixosChannels : JSON.parse(process.env.NIXOS_CHANNELS)

to compile into

elasticsearchMappingSchemaVersion: parseInt('42'),
elasticsearchUrl: {
  NODE_ENV: 'production',
  SHELL: '/nix/store/0hx32wk55ml88jrb1qxwg5c5yazfm6gf-bash-5.2-p15/bin/bash',
  // the entire environment in JSON
}.ELASTICSEARCH_URL || '/backend',
elasticsearchUsername: {
  // the entire environment again
}.ELASTICSEARCH_USERNAME || ...,
elasticsearchPassword: {
  // again
}.ELASTICSEARCH_PASSWORD || ...,
nixosChannels: JSON.parse(...)

this removes it and puts the remaining used env vars in the existing EnvironmentPlugin:

new webpack.EnvironmentPlugin({
    ELASTICSEARCH_MAPPING_SCHEMA_VERSION: undefined,
    ELASTICSEARCH_URL: null,
    ELASTICSEARCH_USERNAME: null,
    ELASTICSEARCH_PASSWORD: null,
    NIXOS_CHANNELS: undefined
}),

(https://webpack.js.org/plugins/environment-plugin/#usage-with-default-values undefined for required, null for optional)

so that it now compiles into this less detailed code

elasticsearchMappingSchemaVersion: parseInt("42"),
elasticsearchUrl:  false || '/backend',
elasticsearchUsername:  false || ...,
elasticsearchPassword:  false || ...,
nixosChannels: JSON.parse(...)