invoice processing webapp

need core knockout JS full stack

Also, please ensure that the invoice can still be saved even if client = null or project = null

Please test on some old invoices .. I want to make sure this is still backward-compatible. Also on an old invoice:
ordinarily, the Save button is disabled for an old invoice; there is a button called "Enable Save", which re-enables the Form for saving
Please make sure that, when click "Enable Save"
client cannot be changed (as is previously)
project can be changed