Option to redact emailed copies of any case activity (sent to assignees or to other contacts with case roles)

Rationale
========
When users send a copy of a case activity to Assignees and / or to other contacts with roles in the case via the Activities UI - the emails contain potentially sensitive information about the client and the case. Organizations using CiviCase need to the option to force this data to be redacted to prevent this information from being exposed while in transit through the email delivery network.

Implementation
==============
1. Add a new element in the CiviCase settings file : CRM/Case/xml/configuration/Settings.xml, something like this (feel free to modify structure if there's an easier approach).

<Settings>
...
<RedactActivityEmail>1</RedactActivityEmail>
</Settings>

• default value is 0 (false) if this setting is not present

2. Check this configuration setting for:

• Send assignee copy of actiivty
• Send a copy to case role(s) ... feature (from within activity create / edit)

NOTE: I think both are handled by this method: CRM_Case_BAO_Case::sendActivityCopy()

If setting is TRUE, redact all output values (including client name) using the configured redaction rules for that site.