FEATURE: add user opt to select bookmarks as home (#10597)

Admins can currently add the bookmarks discovery route link
to the homepage interface, but users can't presently select
that as their default home view.  This change facilitates that,
adding the option to the existing Default Home Page dropdown on
the User Preferences Interface page.

test/javascripts/helpers/site.js

@@ -29,7 +29,15 @@ PreloadStore.store("site", {
     { id: 20, name: "ubuntu" },
     { id: 21, name: "test" },
   ],
-  filters: ["latest", "unread", "new", "starred", "read", "posted"],
+  filters: [
+    "latest",
+    "unread",
+    "new",
+    "starred",
+    "read",
+    "posted",
+    "bookmarks",
+  ],
   periods: ["all", "yearly", "monthly", "weekly", "daily"],
   top_menu_items: [
     "latest",