Add User and change API to return only data of current logged user

[Rebolon]

• Add User and change API to return only data of current logged user. Except if it's an admin that can have access to everything. It's more like real app where data access must be restricted to user authorizations
• Add Friends
• Add a Loan system, where you can Loan/Borrow a book to/from a Friend, with a notification system based on remindDate

[Rebolon]

The work is in progress here: #119