R1:957c1d6602a5

Add a setting for selecting SMTP mail encoding

Summary:
Fixes T5956. We changed the default mail encoding to `quoted-printable` to fix delivery via SendGrid via SMTP, but this broke multiple other mailers.

- Change the default back to 8bit (which works everywhere except SendGrid).
- Add a configuration setting for selecting `quoted-printable`.
- Document this issue.
- Discourage use of SendGrid in documentation.

(IMPORTANT) @klimek @nickz This reverts the `quoted-printable` fix for SendGrid. You…