VAR-1: Add variable

[albertosml]

As a user, I want to add a variable, so it can be used on any template field expression and processed when generating an invoice.

To do that, the user will introduce a name, a description, a entity attribute to map the variable and the associated subtotal if the entity attribute is the invoice subtotal.

Respect to the UI, this process will be done through a form for the registration.

Dependencies

• SUBT-1: The variable can be associated to a variable, so it has to exist.
• SUBT-2: Subtotals needs to be listed before being associated to a variable.