This commit tweaks the landing page to make various interrelated
improvements:
- Improve alignment by moving the quick KCM link buttons into the
  FormLayout
- Use a 2x3 grid for the "Most Used" icons and improve the code quality
- Improve spacing by adding some vertical padding in relevant places
- Improve clarity by putting each radio button's explanatory text right
  below it

| Before | After |
| ------ | ----- |
| ![SySe_before](/uploads/4b554745da5ab3555ce233af8519ae7b/SySe_before.png) | ![SySe_after](/uploads/b3c8e7a43ea1d57135b58c7032212b7f/SySe_after.png) |

cc @teams/vdg