Extract placeholder selector for button styles

So we can reuse it.

app/assets/stylesheets/layout.scss

@@ -91,12 +91,8 @@ a {
 }
 
 .button {
+  @extend %button;
   background: $brand;
-  font-size: $base-font-size;
-
-  &:hover {
-    text-decoration: none !important;
-  }
 
   &.warning,
   &.warning:hover {

app/assets/stylesheets/mixins.scss

@@ -199,17 +199,21 @@
   }
 }
 
-@mixin hollow-button($color: $link) {
-  @include button($style: hollow, $background: $color);
+%button {
   font-size: $base-font-size;
-  margin-bottom: 0;
 
   &:focus,
   &:hover {
-    text-decoration: none;
+    text-decoration: none !important;
   }
 }
 
+@mixin hollow-button($color: $link) {
+  @include button($style: hollow, $background: $color);
+  @extend %button;
+  margin-bottom: 0;
+}
+
 @mixin header-font-size($heading-tag) {
   @each $size, $headers in $header-styles {
     @include breakpoint($size) {