feat(admin-email): create template tag to pass the donor comment to email templates

[samsmith89]

User Story

As a site admin, I want to have a Donor Comment template tag to place in the email templates so that I can have that information sent in emails.

Current Behavior

I currently can't do this as this information isn't passed in the Give API to create a {meta_donation} template tag.

Expected Behavior

I expect to pass this information on to the outbound emails.

Related

https://secure.helpscout.net/conversation/655641095/24711?folderId=1823994