Set up emails to handle Google 'Email Markup'

[karbassi]

Allows Google Mail (Gmail or Google Apps) users to do quick actions showcased here:

https://developers.google.com/gmail/markup/

• [ ] 'RSVP' feature for volunteers. [source]
• [ ] 'Add to Calendar' feature for volunteers. [source]
• [ ] 'Add to Calendar' feature for guardians.
• [ ] 'Review' class feature for volunteers.
• [ ] 'Review' class feature for guardians.

Example Screenshots: