Define Project session form and other stuff

Summary:

- New structure that includes Sessions (to create a new session go to localhost:3000/sessions)
- Arrangements to relate a Session to a ProjectSession 
- Incorpore the ProjectSession form to the project form
- Fixes based on the ['To-do' list](https://github.com/ipoconnection/ipo-web/wiki/To-do-list-(for-developers-only))
- First approach for a many-to-many relationship between `Student`and `Reference`
- Use SettingsLogic for the application deadline value
- Handle authentication in projects & students form