fix(forms): Fix contrast issues on forms

* .help-block has a darker color
* toggle labels are darker

BREAKING CHANGES (scss) .help-block has a darker color now. For the old
color, use $gray-dark.

src/pivotal-ui/components/forms/forms.scss

@@ -1117,6 +1117,10 @@ Here's an example in a vertical form:
   text-transform: capitalize;
 }
 
+.help-block {
+  color: $form-label-color;
+}
+
 // Form control sizing
 
 @include input-size('.input-sm', $input-height-small, $padding-small-vertical, $padding-small-horizontal, $font-size-small, $line-height-small, $border-radius-small);
@@ -1273,6 +1277,7 @@ A toggle switch is a horizontally styled checkbox which represents true with blu
     position: relative;
     display: block;
     background-color: $neutral-7;
+    color: $neutral-1;
     transition: background 0.4s;
     cursor: pointer;
     outline: none;

src/pivotal-ui/components/pui-variables.scss

@@ -514,6 +514,8 @@ $input-border-width:             1px !default;
 
 $input-color-placeholder:        $neutral-5 !default;
 
+$form-label-color:               $neutral-3 !default;
+
 $input-height-base:              ($line-height-computed + ($padding-base-vertical * 2) + 2) !default;
 $input-height-large:             (floor($font-size-large * $line-height-large) + ($padding-large-vertical * 2) + 2) !default;
 $input-height-small:             (floor($font-size-small * $line-height-small) + ($padding-small-vertical * 2) + 2) !default;