Integrate users and books

- new table userbook linking users and book
- moved rating to users book
- updated demo data and tests
- updated front to hide routes from non connected users

internal/model/userbook.go

@@ -0,0 +1,12 @@
+package model
+
+import "gorm.io/gorm"
+
+// describes the relationship between a user and a book.
+type UserBook struct {
+	gorm.Model
+	UserID uint
+	BookID uint
+	Book   Book
+	Rating int
+}