UX: Improve share modal consistency (PR #13689)

Came here to fix some Safari alignment (Topic share modal buttons label alignment issue - #3 - ux - Discourse Meta), but cleaned up some other things along the way:

  • All buttons are now buttons and use the default button styles
  • If everything is btn-primary, nothing is btn-primary — I think the copy button being primary makes the most sense here
  • Added some brand colors
  • Fixed alignment
  • Removed some excess bottom space

Before:

Screen Shot 2021-07-09 at 6 15 40 PM

After: Screen Shot 2021-07-09 at 8 27 19 PM

GitHub

Looks like the tests need to be updated though.