Invoices carry an Attention line

The printed invoice's Bill To block showed the GC's company and address but no person. Each invoice now carries an Attention field — set it in the invoice editor and the printed invoice reads "Attn: …" right under the company name.

Type it once per project: new invoices (including the monthly batch) inherit the attention line from the project's latest invoice. Like the rest of the issued content, it locks when the invoice is sent.