Implement 'introducer remove'

- Implement the subcommand `remove` for `introducer`.
- Remove the introducer by adding a certification with trust level 1 and
  trust depth 0.
- Move `crypt.rs` into `common/` and add a generic authenticaton
  function to be used by `introducer add` and `... remove`.

src/commands/introducer/remove.rs

@@ -1,6 +1,7 @@
 use anyhow::Result;
 
 use sequoia_openpgp::{Fingerprint, KeyHandle};
+use sequoia_openpgp::cert::ValidCert;
 use sequoia_cert_store::Store;
 
 use crate::cli::cli_args::IntroducerRemoveCommand;
@@ -23,6 +24,19 @@ pub async fn dispatch(cmd: IntroducerRemoveCommand, config: HuskConfigContainer)
     let certs = cert_store.lookup_by_cert(&KeyHandle::try_from(&fpr)?)
         .map_err(|_| CommandError::CertNotFound(fpr.clone()))?;
 
+    // Filter out valid certificates. If the certificate is not
+    // valid according to the policy, it is already no introducer.
+    let certs: Vec<ValidCert> = certs.iter()
+        .filter_map(|c|
+            c.with_policy(policy, None).ok()
+        )
+        .collect();
+
+    if let Some(cert) = certs.first() {
+        Introducer::remove(&context, cert, cmd.demote)?;
+    } else {
+        return Err(CommandError::CertNotFound(fpr).into());
+    }
 
 
     Ok(())