Add footer

- Add a footer to each mail resulting from a split into an encrypted and
  an unencrypted version. The footer contains the recipients of the mail
  which are excluded in the corresponding stream.
- Move the construction of the `HuskContext` to the `mail`
  milter-command, as `helo` is used by scrapers to detect mailservers.
- Use `CertD::trust_root()` to get the trust root - it creates one if no
  root can be found.
- Add `chrono` dependency.

src/mail.rs

@@ -11,8 +11,10 @@ use lettre::transport::smtp::{
 use lettre::Address;
 use tokio::task::JoinSet;
 use std::time::Duration;
+use chrono::Utc;
 
 use crate::types::cheader::CHeader;
+use crate::types::defaults;
 use crate::types::errors::HuskError;
 use crate::types::recipient::Recipient;
 
@@ -128,3 +130,55 @@ pub async fn smtp_inject(from: String, to: &Vec<Recipient<'_>>, header: CHeader,
     }
 }
 
+pub fn generate_footer(rcpts: &Vec<Recipient>) -> String {
+    internal_generate_footer(Utc::now().to_rfc2822(), rcpts)
+}
+
+pub(crate) fn internal_generate_footer(now: String, other_rpcts: &Vec<Recipient>) -> String {
+    let mut footer = defaults::FOOTER.replace("{date}", now.as_str());
+
+    if ! other_rpcts.is_empty() {
+        let rcpts = other_rpcts.iter()
+            .map(|r| r.email.clone())
+            .collect::<Vec<String>>()
+            .join(", ");
+        footer.push_str(
+            defaults::FOOTER_RECIPIENTS.replace(
+                "{rcpts}",
+                &rcpts).as_str()
+            );
+    }
+    footer
+}
+
+
+#[cfg(test)]
+pub mod tests {
+
+    use crate::mail::internal_generate_footer;
+    use crate::types::recipient::Recipient;
+
+    #[test]
+    fn test_footer() {
+
+        let rcpts = vec![
+            Recipient {
+                email: "alice@example.com".to_string(),
+                certs: Vec::new()
+            },
+            Recipient {
+                email: "bob@example.com".to_string(),
+                certs: Vec::new()
+            }
+        ];
+
+        let footer = internal_generate_footer(
+            "1.1.1970".to_string(),
+            &rcpts
+        );
+
+        assert!(footer.contains("1.1.1970"));
+        assert!(footer.contains("alice@example.com"));
+        assert!(footer.contains("bob@example.com"));
+    }
+}