Announcements

- [x] Create an `Announcement` model
  - id
  - post_id
  - bulletin_id
  - description
  - position
  - [`acts_as_list`](https://github.com/swanandp/acts_as_list)
- [x] Add a new announcement to a bulletin
```
POST /groups/:group_id/bulletins/:bulletin_id/announcements
```
- [x] Add a new announcement to a bulletin at index
```
POST /groups/:group_id/bulletins/:bulletin_id/announcements/:position
```
- [x] Update an announcement
```
PUT /announcements/:id
```
- [x] Delete an announcement
```
DELETE /announcements/:id
```
- [x] Move announcement to different position
```
PATCH /announcements/:id/move/:position
```

Notes

- 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.