css: Use the Zulip static asset pipeline for emoji sprite sheet CSS.

This should be a nonfunctional change in most cases; the main benefit
should be preventing caching issues if/when our sprite sheets change.

templates/zerver/index.html

@@ -14,7 +14,7 @@ var page_params = {{ page_params }};
 {% block customhead %}
 <meta name="apple-mobile-web-app-capable" content="yes">
 <link href="/static/images/logo/apple-touch-icon-precomposed.png" rel="apple-touch-icon-precomposed">
-<link rel="stylesheet" type="text/css" href="/static/generated/emoji/google_sprite.css" />
+{% stylesheet 'google_sprite' %}
 <style type="text/css">
 
 #css-loading {

zproject/settings.py

@@ -755,6 +755,30 @@ PIPELINE = {
             ),
             'output_filename': 'min/common.css'
         },
+        'apple_sprite': {
+            'source_filenames': (
+                'generated/emoji/google_sprite.css',
+            ),
+            'output_filename': 'min/google_sprite.css',
+        },
+        'emojione_sprite': {
+            'source_filenames': (
+                'generated/emoji/google_sprite.css',
+            ),
+            'output_filename': 'min/google_sprite.css',
+        },
+        'google_sprite': {
+            'source_filenames': (
+                'generated/emoji/google_sprite.css',
+            ),
+            'output_filename': 'min/google_sprite.css',
+        },
+        'twitter_sprite': {
+            'source_filenames': (
+                'generated/emoji/google_sprite.css',
+            ),
+            'output_filename': 'min/google_sprite.css',
+        },
     },
     'JAVASCRIPT': {},
 }