Additional models for all Membership related emails

[maneesha]

All Membership related emails (currently only member onboarding) should include Person and Event models as available fields.

We want to be able to list out the Persons for instuctor training seats used and the Events for workshops used. This is less critical for the onboarding email as no seats or workshops should be used then, but is more critical for the 3/6/9 month emails that will follow.