inputs under <fieldset disabled> not styled as disabled

Created by: cvrebert

In HTML5, <fieldset> gains support for the disabled attribute, allowing you to disable an entire fieldset at once. However, Bootstrap does not seem to style all input elements under disabled fieldsets appropriately; some aren't grayed-out like individually disabled input elements. They should be.

(Re-reporting since #5605 (closed) was somehow erroneously closed as a duplicate of itself.)