adding narrow button mixin

opentech/static_src/src/sass/abstracts/_mixins.scss

@@ -78,7 +78,7 @@
     }
 }
 
-// button mixin
+// Button mixin
 @mixin button($bg, $hover-bg) {
     padding: 10px 60px;
     font-weight: $weight--bold;
@@ -94,6 +94,16 @@
     }
 }
 
+// Narrow button mixin
+@mixin button--narrow {
+    padding: 5px;
+    font-size: 15px;
+
+    @include media-query(tablet-landscape) {
+        padding: 5px 15px;
+    }
+}
+
 // Viewport sized typography mixin that takes a min and max pixel-based value
 @mixin responsive-font-sizes($min, $max) {