Extend core track

[sshine]

Before we move the SML track to v3, let's move it to v2.

This commit creates the following order of core exercises:

$ jq -r '.exercises[] | select(.core) | .slug' config.json
hello-world
leap
two-fer
space-age
pangram
collatz-conjecture
matching-brackets
allergies

This was not chosen arbitrarily or with great care, but should work first as a first approximation. This will potentially attract more students to the track, since the core exercise overview page is much more attractive than the independent mode overview.