unread_ui: Wait for i18n setup before rendering.

Without this, the "Since you were last here" text got rendered in
English for me every time when I tested in German, both in dev
and on chat.zulip.org.

This brings us to 9 places we invoke `ensure_i18n`.  That seems
like a sign that there may well be more places we're still
missing, and that we should probably find a more systematic way
to make sure all our frontend UI rendering waits for translation
to be ready.  Anyway, for now, fix this one.

static/js/unread_ui.js

@@ -93,7 +93,7 @@ function consider_bankruptcy() {
     }
 }
 
-exports.initialize = function initialize() {
+function _initialize() {
     // No matter how the bankruptcy modal is closed, show unread counts after.
     $("#bankruptcy").on("hide", function () {
         unread_ui.enable();
@@ -108,8 +108,11 @@ exports.initialize = function initialize() {
     });
 
     consider_bankruptcy();
-};
+}
 
+exports.initialize = function () {
+    i18n.ensure_i18n(_initialize);
+};
 
 return exports;
 }());