Implement confirmation email.

[jcadam14]

Implement the confirmation email sending. See cfpb/sbl-ux#23 (comment) for the text of the email.

This will take in the contact email, the signer email, the signer user name, the confirmation id, and the signature timestamp (see cfpb/sbl-filing-api#495)

Use the existing /send endpoint for an example on how to build the email object