Fix linear-gradient syntax

As explained at https://developer.mozilla.org/en-US/docs/Web/CSS/linear-gradient#History_of_the_syntax
the direction syntax was changed a long time ago from '<starting point>'
to 'to <destination point>'.

This fixes a warning from autoprefixer.

h/static/styles/forms.scss

@@ -229,7 +229,7 @@
 }
 
 .btn-danger {
-  @include background(linear-gradient(top, $error-color, shade($error-color, 5%)));
+  @include background(linear-gradient(to bottom, $error-color, shade($error-color, 5%)));
   color: white;
   border-color: shade($error-color, 15%);
   text-shadow: 0 -1px 0 rgba(0, 0, 0, .1);

h/static/styles/variables.scss

@@ -34,7 +34,7 @@ $brand-color: #bd1c2b !default;
 $button-text-color: $gray-dark !default;
 $button-background-start: $white !default;
 $button-background-end: #f0f0f0 !default;
-$button-background-gradient: top, $button-background-start, $button-background-end !default;
+$button-background-gradient: to bottom, $button-background-start, $button-background-end !default;
 
 $error-color: #f0480c !default;
 $success-color: #1cbd41 !default;