timestamp: Migrate message_time from title to tippyjs tooltip.

This keeps it consistent with other widgets in message body area.

Set the display position to top to be consistent with
compose control buttons.

Changed the tooltip content to be more readable like
Thursday, May 18, 2017
7:12:53 AM India Standard Time

Also changed timerender.get_full_datetime() to consider
users' 24 hour format preference.

static/js/tippyjs.js

@@ -145,4 +145,14 @@ export function initialize() {
             return true;
         },
     });
+
+    delegate("body", {
+        target: ".message_time",
+        allowHTML: true,
+        placement: "top",
+        appendTo: () => document.body,
+        onHidden(instance) {
+            instance.destroy();
+        },
+    });
 }