This change fixes two things with the list edit widget, which is used

for the diffviewer syntax highlighting file type override configuration.


In the case that the widget is used within a ConfigForms page, the

   30em width for input fields was causing the widget to bust out of the

   60em width of the config forms page. ConfigForms is desperately in

   need of a comprehensive redesign, but for now I've added some style

   overrides to remove the 30em width and instead apply flex-grow: 1.


This change also swaps out the btn class for ink-c-button, giving

   us a better display when running in dark mode. While doing this, I've

   removed role="button" from the element, since that's implicit for

<button> tags.

Viewed the widget in the context of a ConfigForms page and saw that it

  was now fitting nicely into the width of its container, and the "Add

  item" button looked nice in both light and dark modes.
Ran js-tests.