Fix the noise background on the stream and sidebar

 - Correct the image reference to be valid with the new build
   system, where styles are served at /assets/styles and
   images under /assets/images.

 - Use @include rather than @extend per current CSS
   conventions for H

h/static/styles/annotator/inject.scss

@@ -99,7 +99,7 @@ $base-font-size: 14px;
   // frame styles
   @import './bucket-bar';
   @include user-select(none);
-  @extend .noise;
+  @include noise-background;
   direction: ltr;
   background: none;
   font-size: $base-font-size;

h/static/styles/app.scss

@@ -33,7 +33,7 @@ $base-line-height: 20px;
 body {
   $sidebar-h-padding: 9px;
 
-  @extend .noise;
+  @include noise-background;
   font-family: $sans-font-family;
   font-weight: 300;
   padding: $sidebar-h-padding;

h/static/styles/base.scss

@@ -26,8 +26,8 @@
 
 //NOISE///////////
 //Provides the noise background
-.noise {
-  background: image-url("images/noise_1.png");
+@mixin noise-background {
+  background: url("../images/noise_1.png");
 }
 
 .annotator-hide {

h/static/styles/site.scss

@@ -26,7 +26,7 @@
 // body with grey noise background and Lato font used
 // in the sidebar and groups pages
 .body--default {
-  @extend .noise;
+  @include noise-background;
   font-family: $sans-font-family;
 }