CRM-20420 - Generate combined or separated activities, per recordGeneratedLetters

CRM-20420 - Generate combined or separated activities, per recordGeneratedLetters

When a user uses "Print/Merge Document" to generate a PDF or Word doc,
they may designate multiple contacts to include in the PDF/Doc. By
setting `recordGeneratedLetters`, they can specify whether to:

 * Create one activity for each contact. (This is the traditional behavior,
   but when you retain the fully-generated PDF documents, it wouldn't
   be a good idea to copy the PDF to all those activities.)
 * Create a single activity associated with all the contacts.