Part of #506.

Possible improvements would be: - [ ] make this work with JS magic (i.e. no extra page load) - [ ] have the descriptions be hidden by default after a certain (TBD) breakpoint.

Here's how it's implemented in D8 and how it looks in narrow screens:

d8_config_page-show_descriptions

d8_config_page-hide_descriptions

GitHub Issue #: 
979