switch to Sections model, store GitHub data in repo


All this has been merged to dev, and tested there by Erin, Dan and Erika. Ready for live, with a couple of important notes.

BEFORE DEPLOYMENT
-----------------
We need to run the category migration one more time, to pick up any articles that have been added since last week:

- `python manage.py migrate_article_categories`

POST DEPLOYMENT
---------------
Now that GitHub data will be stored locally, we need to run these three commands right after deployment:

- `python manage.py update_code_github_stats`
- `python manage.py update_person_github_stats`
- `python manage.py update_organization_github_stats`

Those will fetch the initial data that the templates need in order to display GitHub info.

And then as soon as is reasonable, we should put those three GitHub commands (as well as the Twitter update command) on a cron.