Replace #stream_sidebar_{id} with data-stream-id.

We weren't actually using #stream_sidebar_{id} anywhere, and
data-stream-id allows for more conventional idioms in future
commits.

static/templates/stream_sidebar_row.handlebars

@@ -1,7 +1,7 @@
 {{! Stream sidebar rows }}
 
 <li data-name="{{name}}" class="narrow-filter{{#if not_in_home_view}} out_of_home_view{{/if}}"
-    id="stream_sidebar_{{id}}">
+    data-stream-id="{{id}}">
     <div class="subscription_block selectable_sidebar_block" data-name="{{name}}">
 
         <span id="stream_sidebar_privacy_swatch_{{id}}" class="stream-privacy" style="color: {{color}}">