Make sysadmin dashboard gitlog date timezone-aware

@carsongee Just a small change to the sysadmin dashboard gitlog:
* Redundancy reduced in the template
* Date is timezone aware and formatted as `util.date_utils.DEFAULT_DATE_TIME_FORMAT`

Before:
-----------
![before](http://i.imgur.com/71heogf.png)

After:
--------
![after](http://i.imgur.com/GnBQvzQ.png)


I know you didn't actually modify this line, but as long as it's changing, can you improve this translation string by giving some more context to the translators? I'd suggest doing:
```mako
<h2>${_('Recent git load activity for {course_id}').format(course_id=course_id)}</h2>
```