WIP receive emails and invoke `email-lambda` to post message on relevant pinboard

[twrichards]

totally a WIP, currently blocked on being able to create the MX DNS record via GuCDK...

• https://github.com/guardian/cdk/blob/e95d839b070e1e00f606d94b4d608a591b768af2/src/constructs/dns/dns-records.ts#L7-L9
• https://github.com/guardian/cfn-private-resource-types/blob/a175adf7fc91452d36b7427e265e0ee4c875e2bd/dns/dns-change-executor/src/validation.ts#L19-L23
• https://github.com/guardian/cfn-private-resource-types/blob/a175adf7fc91452d36b7427e265e0ee4c875e2bd/dns/dns-change-executor/src/validation.ts#L70-L73

TODO

• [ ] configure MX DNS record via GuCDK
• [x] configure SES rule to invoke email-lambda
• [ ] have email-lambda handle the SES email payload (distinct from the two current ways of invoking)
• [ ] connect email-lambda to AppSync
• [ ] resolve/transform users email address to @guardian.co.uk variant
• [ ] call createItem mutation (rather than DB insert) to ensure clients who are 'subscribed' get real-time message
• [ ] update architecture diagram