Allow localization of month-day format string

The short month-day format ex: 'Aug 8', is not built-in to momentjs, so
we need to localize it.

// FREEBIE

_locales/en/messages.json

@@ -320,5 +320,9 @@
     "timestamp_h": {
        "description": "Brief timestamp for messages sent about one minute ago. Displayed in the conversation list and message bubble.",
        "message": "1 hour"
+    },
+    "timestampFormat_M": {
+       "description": "Timestamp format string for displaying month and day (but not the year) of a date within the current year, ex: use 'MMM D' for 'Aug 8', or 'D MMM' for '8 Aug'.",
+       "message": "MMM D"
     }
 }

js/views/timestamp_view.js

@@ -53,7 +53,7 @@
                 return timestamp.format(this._format.y);
             } else if (timediff.months() > 0 || timediff.days() > 6) {
                 this.delay = null;
-                return timestamp.format(this._format.mo);
+                return timestamp.format(this._format.M);
             } else if (timediff.days() > 0) {
                 this.delay = moment(timestamp).add(timediff.days() + 1,'d').diff(moment());
                 return timestamp.format(this._format.d);
@@ -78,8 +78,8 @@
             return moment.duration(number, string).humanize();
         },
         _format: {
-            y: "MMM D, YYYY",
-            mo: "MMM D",
+            y: "ll",
+            M: i18n('timestampFormat_M') || "MMM D",
             d: "ddd"
         }
     });
@@ -88,8 +88,8 @@
             return moment.duration(-1 * number, string).humanize(string !== 's');
         },
         _format: {
-            y: "MMM D, YYYY LT",
-            mo: "MMM D LT",
+            y: "lll",
+            M: (i18n('timestampFormat_M') || "MMM D") + ' LT',
             d: "ddd LT"
         }
     });