Store email body instead of headers in RawNotification.raw

[chadell]

Environment

• nautobot-circuit-maintenance version: 0.4.3

Proposed Functionality

When storing raw data from an email notification, we could prioritize the body instead of the headers (size is limited to 16KB)

Use Case

Storing the body instead of the headers (that are a lot in some cases), would help to troubleshoot why a notification parser failed.

[glennmatthews]

Looks like a case we missed in #113.