Announcements

- [x] Create an `Announcement` model
  - id
  - post_id
  - bulletin_id
  - description
  - position
  - [`acts_as_list`](https://github.com/swanandp/acts_as_list)
- Bulletins will be preloaded with: (To be implemented in frontend repository)
  - posts tagged with `announcement`
  - copy announcement description from previous week's bulletin

This will fix #6.