Add blurb to .meta/config.json files

[ErikSchierboom]

Each Concept and Practice Exercise will have to define a blurb, which is a short description of the exercise. The blurb will be displayed on a track's exercises page (e.g. https://exercism.lol/tracks/csharp/exercises).

For Practice Exercises that are based on an exercise defined in the problem-specification repo, the blurb must match the contents of the problem-specifications exercises, which is defined in its metadata.yml file. We'll do an initial syncing of the blurb. The new configlet version will add support for doing this syncing automatically.

If the Practice Exercise was not based on a problems-specifications exercise, we'll use the blurb from its .meta/metadata.yml file as the blurb in the .meta/config.json file.

For Concept Exercises, we'll add a placeholder blurb to the .meta/config.json file of each Concept Exercise.

[ErikSchierboom]

All PR's have been merged.