Set subject to Re: <last subject> for better compability with normal MUAs

The code in dc_receive_imf.rs looks a bit funny, an alternative would be a function:

fn upcate_chat_last_subject(context: &Context, chat_id: &ChatId, mime_parser: &mut MimeMessage) -> Result<()> {
    let mut chat = Chat::load_from_db(context, *chat_id)?;
    chat.param.set(Param::LastSubject, mime_parser.get_subject().ok_or_else(||Error::Message("No subject in email".to_string()))?);
    chat.update_param(context)?;
    Ok(())
}

src/dc_receive_imf.rs

@@ -10,7 +10,7 @@ use crate::constants::*;
 use crate::contact::*;
 use crate::context::Context;
 use crate::dc_tools::*;
-use crate::error::{bail, ensure, Result};
+use crate::error::{bail, ensure, format_err, Result};
 use crate::events::Event;
 use crate::headerdef::HeaderDef;
 use crate::job::{self, Action};
@@ -749,6 +749,25 @@ async fn add_parts(
         }
     }
 
+    || -> Result<()> {
+        let mut chat = Chat::load_from_db(context, *chat_id)?;
+        chat.param.set(
+            Param::LastSubject,
+            mime_parser
+                .get_subject()
+                .ok_or_else(|| format_err!("No subject in email"))?,
+        );
+        chat.update_param(context)?;
+        Ok(())
+    }()
+    .unwrap_or_else(|e| {
+        warn!(
+            context,
+            "Could not update LastSubject of chat: {}",
+            e.to_string()
+        )
+    });
+
     Ok(())
 }