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 will need to adjust your configurations (set `phpmailer.smtp-encoding` to `quoted-printable`) and restart your daemons or mail will get double newlines again.

Test Plan:
  - Sent mail via SendGrid with various `phpmailer.smtp-encoding` settings, saw mail arrive with specified encoding.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: klimek, nickz, epriestley

Maniphest Tasks: T5956

Differential Revision: https://secure.phabricator.com/D10397