[cms] Allow to edit invoice that is waiting for payment

[xaur]

It is my understanding that CMS currently does not allow to edit an invoice once it has entered the Waiting for payment state. This is problematic because, as much as everyone tries to do their part correctly, human errors do and will happen, and software should allow to correct them.

If an invoice line item uses the wrong proposal token Y instead of token X, it somehow passes review and gets paid, we gain two accounting bugs:

• owner of proposal X will not see the expense in the summary for proposal X and may report an incorrect (lowered) expense to the stakeholders

• owner of proposal Y will get surprised by an unexpected expense and may report an incorrect (increased) expense to the stakeholders

It would be good if invoices with wrong tokens could go back from Waiting for payment to Waiting for review and get corrected.