Use the new design for checkboxes and fix alignment.
Removes redesigned-input, which is a small regression on the old design, but I think it's acceptable bcause we're going to shut it down soon.
There were all sorts of dependencies hiding in there, and it was overwriting some of the admin variables. Better to keep things separate and simple.
I discovered that there was one legitimate reason for importing them though: for styling the trix editor. I can't think of a good way to safely import them without affecting the other variables, so I just hardcoded the colours for now.
