Sign encrypted mails.

- Husk now not only encrypts outgoing emails (if possible) but also adds
  a signature.
- Rework the internal handling of the signing key.
- Integrate the signing into the workflow of the daemon.
- Add a configuration option for a subject replacement.

src/types/defaults.rs

@@ -1,9 +1,13 @@
 //! Defaults
 
-pub const CONFIG_FILE_LOCATION: &'static str = "/etc/husk/config.toml";
+pub const CONFIG_FILE_LOCATION: &str = "/etc/husk/config.toml";
+
+/// If encrypting a mail, the subject is moved into the encrypted body
+/// and replaced by `SUBJECT_REPLACEMENT` (unless configured otherwise).
+pub const SUBJECT_REPLACEMENT: &str = "...";
 
 /// Default logging configuration.
-pub const CONSOLE_LOGGING: &'static str = "
+pub const CONSOLE_LOGGING: &str = "
 [appenders.console]
 kind = \"console\"