[Overview] Improve existing Concept Exercises

[junedev]

This is a tracking issue for the concept exercises that need some cleanup or improvement.

• [x] https://github.com/exercism/javascript/issues/1049 (claimed)
• [x] https://github.com/exercism/javascript/issues/1166
• [x] https://github.com/exercism/javascript/issues/1247
• [x] https://github.com/exercism/javascript/issues/1325
• [x] https://github.com/exercism/javascript/issues/1326 (claimed)
• [x] https://github.com/exercism/javascript/issues/1373
• [x] https://github.com/exercism/javascript/issues/1340
• [x] https://github.com/exercism/javascript/issues/1341
• [x] https://github.com/exercism/javascript/issues/1373
• [x] https://github.com/exercism/javascript/issues/948
• [ ] https://github.com/exercism/javascript/issues/945
• [ ] https://github.com/exercism/javascript/issues/1634
• [ ] https://github.com/exercism/javascript/issues/1529
• [ ] https://github.com/exercism/javascript/issues/1234 (low priority)

There are probably some more issues needed, feel free to create additional ones if you notice problems with existing concept exercises.