Mark exercises as core

[mawis]

Relates to #92

Where the v1 site has a long, linear list of exercises, the v2 site has organized exercises into a small set of required exercises ("core").

If you update the track config, remember to get configlet and run configlet fmt . from the root of the track before submitting.

[mawis]

All our exercises are currently "core": false. We have to select and arrange the core exercises.

@kytrinyx already suggested, that “Bob at third place might be bad, it could be to difficult” from experience in other tracks.

She also suggests, that “mathy, comp sci, algorithm exercises tend to not be compelling to people”. We might configure these as side exercises.