Make sunlight_announcements functionality more generic

* Alias `organization_announcements` to `sunlight_announcements` in Mongoid, i.e. the field name in MongoDB is still `sunlight_announcements` to avoid breaking existing databases, but the field name everywhere else in the code is now `organization_announcements`.
* Add `organization` section to `config.yml`, with `name` and `url` keys. To upgrade, simply add this section to your production `config.yml` file. I haven't used these new configuration variables everywhere - I've just updated the parts related to the announcements functionality.