Fix inconsistent input[disabled] color in Safari

Fixes #2355 

Safari is rendering `disabled` input fields with faded text. 

Instead of the Bulma default `#7a7a7a` (48% luminosity), the input text renders at `#cecece` (81% luminosity).

Multiple users reported confusion, thinking the `<input disabled> value=` was instead `placeholder=` text, because it was so faded.

Bulma is [already choosing to override the default `<input disabled>` text `color`](ff8c4c18ac/sass/elements/form.sass (L67)). That setting simply isn't taking effect in Safari & iOS Safari.

This PR fixes it.

sass/form/shared.sass

@@ -48,6 +48,8 @@ $input-radius: $radius !default
     border-color: $input-disabled-border-color
     box-shadow: none
     color: $input-disabled-color
+    -webkit-text-fill-color: $input-disabled-color
+    opacity: 1
     +placeholder
       color: $input-disabled-placeholder-color