Fix unclickable checkboxes (#30195) (#30199)

Fix unclickable checkboxes (#30195) (#30199)

Backport #30195 by @silverwind

Fix https://github.com/go-gitea/gitea/issues/30185, regression from
https://github.com/go-gitea/gitea/pull/30162.

The checkboxes were unclickable because the label was positioned over
the checkbox with `padding`. Now it uses `margin` so the checkbox itself
will be clickable in all cases.

Secondly, I changed the for/id linking to also add missing `for`
attributes when `id` is present. The other way around (only `for`
present) is currently not handled and I think there are likey no
occurences in the code and introducing new non-generated `id`s might
cause problems elsewhere if we do, so I skipped on that.

Co-authored-by: silverwind <me@silverwind.io>