User needs a way to delete their account #272

The assumptions made are:

1) Privately owned apps should be cascade deleted
2) Publicly owned apps should remain, but be unowned
3) All OAuth Tokens and Grants should be cascade deleted

- Updated some HTML on the Profile page
- Added a delete button on the Profile page
- Created a screen and action for entering the email and destroying the profile
- Added tests to cover UI and to check for models being deleted
- Changed the user model to cascade delete private apps and to nullify public apps
- Changed the user model to cascade delete OAuth authorizations
Update the warning message
Screen updates and auto-delete public apps

- Displays a list of private and/or public apps to be deleted
- Deletes public apps rather than disassociate from user
- Changed the "Enter your email" prompt
Merge branch 'devel' into feature/destroy-account
Fix text
Update test to check for warning messages
Copy change
Change location of warning message
Merge pull request #282 from 18F/feature/destroy-account

User needs a way to delete their account