Decide on track curriculum by making use of "unlocked_by" and rearranging exercises in config.json

[ilya-khadykin]

In https://v2.exercism.io/ a different approach of showing exercises was chosen.
Now it's very important to plan ahead what particular exercise unlocks and their order.

You also expected to choose 'core' exercises creating a track curriculum.

The only limitation I remember, non-core exercises should be only unlocked by core ones, not the other way around