R1:c3cafffed726

Update the "SES" and "sendmail" mailers for the new API; remove "encoding"

Summary: Ref T13222. Ref T920. This is the last of the upstream adapter updates.

Test Plan:
- Sent mail with SES.
- Sent mail with "sendmail". I don't have sendmail actually configured to an upstream MTA so I'm not 100% sure this worked, but the `sendmail` binary didn't complain and almost all of the code is shared with SES, so I'm reasonably confident this actually works.

Reviewers: amckinley

Reviewed By: amckinley…