Rename, don't delete PDFs once the invoices/proformas are issued

[zalmanu]

Once an invoice/proforma is issued, don't delete its corresponding PDF, just rename it to something like INV...-deleted.pdf. This will prevent complete data loss if an invoice/proforma is deleted by mistake.

[presslabs-bot]

This issue will be automatically closed on Tue, 17 Jan 2017 due to the lack of activity (comments or updates). Closed issues that remain inactive for a long period of time might also get automatically locked.

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cc: @calind, @pdobrescu, @vtemian