Fix ERD fields

[Ramin-RX7]

Though the ERD is implemented in #8 there are still some problems with the field types

Customer

• customer table must be removed (read order table notes)

Order

• price must be float
• date is not a suitable name for the field
• customer must be a field of a phone number
• add table number field

OrderItem

• rename price with a better name that implies the field is meant to be for a sigle item
• add discount field

Note that all fields must be lowercase All fields nullability must be defined (in 2nd erd that contains more info on fields)

[Ramin-RX7]

completed in #36