Merge pull request #2997 from hypothesis/fix-form-button-gradients

Fix form button background gradient

h/static/styles/mixins/forms.scss

@@ -36,9 +36,9 @@
 }
 
 @mixin btn {
-  @include background(linear-gradient($button-background-gradient...));
   @include box-shadow(0 1px 0 rgba(0, 0, 0, .15));
 
+  background: linear-gradient($button-background-gradient);
   display: inline-block;
   font-weight: bold;
   color: $button-text-color;