Allow retention of generated PDFs/Docs

CiviCRM includes a feature for generating PDF or Word documents with mail-merge tokens. These can be generated from "View Contact", "Manage Case", "Search Tasks", and possibly others.

Each time you generate a document, it creates an activity record (one per contact). However, it does not store a copy of the generated file. The goal of this issue is to store the full document as an attachment to the activity. The change in behavior should be discretionary (based on a setting).