Feature/private bins

- [ ] Ready to merge - closes #982 

Tasks:

- [x] Db update scripts for visibility column
- [x] Db query templates for checking privacy
- [x] Model methods, delegating to store
- [x] method to test if user can see bin
- [x] UI for setting a bin to private
- [x] Db query for setting visibility on bin
- [x] If previous revision is private, make sure future revisions are private
- [x] Implement all db changes for sqlite
- [x] Make sure all compatible with anonymous bins
- [x] Handle newly created bins, via 'easter egg'
- [x] serve default files for private bin that user doesn't have access too