As mentioned in commit 5311daadf, there are several reasons to use
buttons in these situations. And, as mentioned in the previous commits,
using buttons instead of links for actions requiring confirmation will
help us test for accessibility issues.
Now the padding is only applied in two places (administration forms) so
we can apply it just there instead of applying it everywhere and then
removing it in most places. We're using the `column` class here because
it's what's used in the rest of the fields of these forms and because we
haven't defined (yet) general margin/padding rules for the
administration views.
We don't need any row classes anymore because the <body> already has a
maximum width. As for columns, we only have one column in this form, so
we don't need them either. Besides, the form's parent element already
has a padding.
