[cms] Ability to Revoke an invoice

[xaur]

Sometimes an invoice with a wrong Month/Year is submitted by mistake, and then it is impossible to edit the Month or delete such invoice.

To prevent such invoices from getting paid, the currently used workaround is for invoice author to remove all line items but one, zero out the hours in it, change item's description to something like "BAD INVOICE - PLEASE REJECT", and optionally add the same as a comment.

I think inability to delete a bad invoice is a good feature that should stay.

So the best solution I imagine is to allow invoice authors to flag their invoices as Revoked, and disallow such invoices from getting paid (unless the author removes the flag). Revoked invoices should also have a corresponding label in listings so that reviewers and admins can clearly see them.