Fix soft_delete bugs and update one buddy test

When a samson project or a stage is deleted (and the project/stage has associated deploys), we end up with orphaned deploys.  This bug breaks the Current Deploys and Recent Deploys views.  When the Current Deploys view is broken in this way, the contents of the view cannot be seen, and *all* pending production deploys wait indefinitely.

/cc @zendesk/samson @jwswj @pswadi-zendesk 

### TODO

- [x] refactor one buddy_check test
- [x] modify deploys schema to include deleted_at column
- [x] add cascading delete to project, stage, and deploy models

### RISKS
- need to verify that only soft_deletes occur