Store future concepts in separate config

The track-level config.json maps concepts to the exercises that teach
them and practice them.

As part of preparing for the v3 release we added concepts to all the
practice exercises' practices and prerequisites arrays. However, since
many of these concepts have not yet been taught, configlet is
complaining about a mismatch.

This moves any unimplemented concepts out of the data for the practice
exercises and into a separate JSON file.

That will let us write a script in the future to move concepts back
as we implement them.

Closes #1218

Author: kytrinyx