Fix Bug 965823 - Add a custom Twitter timeline widget to the Student Ambassadors landing page

How to test:

1. Install and run `memcached`
2. Add the [Memcached config](https://docs.djangoproject.com/en/dev/topics/cache/#memcached) and [Twitter config](https://webprod.etherpad.mozilla.org/108) in `local.py`
3. Run `./manage.py cron update_tweets` followed by `./manage.py runserver`