settings: Move stream_sorter_toggle CSS to subscriptions stylesheet.

It's only used for subscriptions so it makes more sense for it to
live there.

web/styles/app_components.css

@@ -337,10 +337,6 @@ input::placeholder {
     }
 }
 
-.stream_sorter_toggle {
-    margin-left: auto;
-}
-
 /* -- unified overlay design component -- */
 div.overlay {
     position: fixed;

web/styles/subscriptions.css

@@ -351,6 +351,10 @@ h4.user_group_setting_subsection_title {
         height: var(--settings-overlay-subheader-height);
     }
 
+    .stream_sorter_toggle {
+        margin-left: auto;
+    }
+
     #add_new_subscription {
         height: var(--settings-overlay-header-button-height);
     }