Create new volunteer mailing tokens

In order to provide context-rich transactional emails and scheduled reminders, we need new tokens:

• Project name
• Role name
• Time Scheduled in Minutes (This is duration; CiviVolunteer adds this custom field.)
• End time (This is a calculation of start time + duration.)
• Time Completed in Minutes (The amount of time the contact actually volunteered.)

For an improved admin user experience, it would be useful to provide these tokens as well:

• Volunteer Name (This is basically just putting a new name on an existing token; folks seem to get confused as to which activity contact is the volunteer.)
• Volunteer Beneficiary (See above.)
• Start time (Again, this is a convenience. For a lot of folks Activity Start Time does not connote Volunteer Shift Start Time. Plus it's handy to have all the volunteer-related tokens under one heading rather than having to jump around between contact, activity, etc.)