Updated lettre (and other crates) and workflow.

General:
- Updated several dependancies

Lettre:
- Updateded lettere and the workflow
- Changed encoding to base64
- Convert unix newlines to dos newlines for e-mails.
- Created custom e-mail boundary (auto generated could cause errors)

Tested the e-mails sent using several clients (Linux, Windows, MacOS, Web).
Run msglint (https://tools.ietf.org/tools/msglint/) on the generated e-mails until all errors were gone.

Lettre has changed quite some stuff compared between alpha.1 and alpha.2, i haven't noticed any issues sending e-mails during my tests.

src/error.rs

@@ -49,7 +49,7 @@ use yubico::yubicoerror::YubicoError as YubiErr;
 use lettre::address::AddressError as AddrErr;
 use lettre::error::Error as LettreErr;
 use lettre::message::mime::FromStrError as FromStrErr;
-use lettre::transport::smtp::error::Error as SmtpErr;
+use lettre::transport::smtp::Error as SmtpErr;
 
 #[derive(Serialize)]
 pub struct Empty {}