Use disabled styling for the button part of the custom-file input

Created by: nathanmesser

Disabled styling was added to the custom file input recently, but that didn't cover the button part of the component, so the button looks enabled when the input is disabled.

I think this could be achieved by adding something like:

&::after { opacity: $btn-disabled-opacity; @include box-shadow(none); }

to the
&:disabled { ~ .custom-control-label {

section of the _custom_forms.scss