fixes #5605: style as disabled all form controls & btns under a disabled fieldset

this is a resubmission of #5875, but now against branch 3.0.0-wip
does not exclude elements under a fieldset's legend
issue #6058 is a duplicate of #5605

less/buttons.less

@@ -48,7 +48,8 @@
 
   // Disabled state
   &.disabled,
-  &[disabled] {
+  &[disabled],
+  fieldset[disabled] & {
     cursor: default;
     background-image: none;
     .opacity(65);
@@ -174,7 +175,8 @@ input[type="button"] {
 // Make a button look and behave like a link
 .btn-link,
 .btn-link:active,
-.btn-link[disabled] {
+.btn-link[disabled],
+fieldset[disabled] .btn-link {
   background-color: transparent;
   background-image: none;
   .box-shadow(none);
@@ -190,7 +192,12 @@ input[type="button"] {
   text-decoration: underline;
   background-color: transparent;
 }
-.btn-link[disabled]:hover {
-  color: @grayDark;
-  text-decoration: none;
+.btn-link {
+  &[disabled],
+  fieldset[disabled] & {
+    &:hover {
+      color: @grayDark;
+      text-decoration: none;
+    }
+  }
 }

less/forms.less

@@ -330,21 +330,24 @@ textarea[class*="span"],
 // --------------
 
 // Disabled and read-only inputs
-input[disabled],
-select[disabled],
-textarea[disabled],
-input[readonly],
-select[readonly],
-textarea[readonly] {
-  cursor: not-allowed;
-  background-color: @input-background-disabled;
+input,
+select,
+textarea {
+  &[disabled],
+  &[readonly],
+  fieldset[disabled] & {
+    cursor: not-allowed;
+    background-color: @input-background-disabled;
+  }
 }
 // Explicitly reset the colors here
-input[type="radio"][disabled],
-input[type="checkbox"][disabled],
-input[type="radio"][readonly],
-input[type="checkbox"][readonly] {
-  background-color: transparent;
+input[type="radio"],
+input[type="checkbox"] {
+  &[disabled],
+  &[readonly],
+  fieldset[disabled] & {
+    background-color: transparent;
+  }
 }
 
 

less/mixins.less

@@ -416,7 +416,7 @@
   .reset-filter();
 
   // in these cases the gradient won't cover the background, so we override
-  &:hover, &:active, &.active, &.disabled, &[disabled] {
+  &:hover, &:active, &.active, &.disabled, &[disabled], fieldset[disabled] & {
     color: @text-color;
     background-color: @endColor;
   }