Admin plugin dirties environment overrides

It was suggested in #27 to make a selection of which config should be updated when changing configurations in the admin. It appears to only have gone as far as displaying which config will be edited.

It seems like a pretty legitimate use-case to edit production/default environment configs in a development environment.

There is another problem. The grav docs on environment config ( https://learn.getgrav.org/advanced/environment-config) state, "Then you can override this with a setting that _**only overrides specific options**_ that you want to use for local testing:".
This works fine, but if you go and use the Admin plugin to change configurations, then all configuration options are written to the environment-specific config..

In summary, either:
* Prevent admin from adding configs to environment-specific configs. The UX could be to disable editing of any configs that are not declared in the active configuration. Some logic would be needed determine which configs are okay to write to file.
* Allow editing of only the default config, i.e. do not allow editing any environment-specific configs with the Admin and allways update the default.
* Provide the ability to select which environment config should be updated in the Admin UI.

Start by reading Grav's environment config documentation and tracing the Admin plugin's configuration-editing flow. Compare the proposed behaviors for default and environment-specific configs; done means the Admin no longer writes unintended full configurations or provides a clear environment selection.

Written by the indexing model from the issue text.