Create a note feature

comments should be detailed enough, but here's a summary of the changes:

1.  Adjusted the views for notes creation and notes review according to your wireframes
2. Created lender and borrower models inheriting from the User model with a Single Table Inheritance pattern
3. Added relationships between lender/borrower and notes which allowed the creation of nested form fields in the notes form
4. Added basic specs to test notes creation and review

To be done : 

1.  think about clarifying the notes creation procedure - who can start the process, the borrower, the lender?, maybe one of them creates the notes and invites the other party to complete the form
2.  think about the signature procedure (maybe show different form fields according to the type of user logged in, lender or borrower)