Adjust order of exercises

[juhlig]

The exercises list-ops and simple-linked-list were placed last in config.json, so they appeared at the end of the exercise list in the track, though they are rated with difficulties 2 and 3, respectively, and the other exercises are ordered by difficulty.

[NobbZ]

Can we agree on put this on hold until #412 got merged to avoid further conflicts with its changes to the config? Then we can order all of them at once.

[juhlig]

Can we agree on put this on hold until #412 got merged to avoid further conflicts with its changes to the config? Then we can order all of them at once.

Of course :) So I'll close this PR for now, and reopen later with the changes from #412 included.