Manage and document environment variables

The way environment variables are handled by Vue have changed from Vue CLI 2 to Vue CLI 3. Now we have .env.production and .env.development, and they are loaded separately depending on the build mode. So you cannot only put those variables in .env.development that are different from those in .env.production, but you have to put all of them there. Only local modifications can then be overloaded with .env.development.local.

So what is to do:

• make a generic .env.development
• add local modifications (like the OIDC client ID) in .env.development.local
• document all of this accordingly in README.md