Use 0 value for "delete_server_after" default.

Now 0 means "never delete", 1 means "delete at once" and other values
indicate the number of seconds after which them message should be
deleted from the server.

Configuration value interpretation is moved into
Context.get_config_delete_server_after() function.

src/config.rs

@@ -68,9 +68,12 @@ pub enum Config {
     /// Timer in seconds after which the message is deleted from the
     /// server.
     ///
-    /// Equals to -1 by default, which means the message is never
+    /// Equals to 0 by default, which means the message is never
     /// deleted.
-    #[strum(props(default = "-1"))]
+    ///
+    /// Value 1 is treated as "delete at once": messages are deleted
+    /// immediately, without moving to DeltaChat folder.
+    #[strum(props(default = "0"))]
     DeleteServerAfter,
 
     SaveMimeHeaders,
@@ -136,6 +139,18 @@ impl Context {
         self.get_config_int(key) != 0
     }
 
+    /// Gets configured "delete_server_after" value.
+    ///
+    /// `None` means never delete the message, `Some(0)` means delete
+    /// at once, `Some(x)` means delete after `x` seconds.
+    pub fn get_config_delete_server_after(&self) -> Option<i64> {
+        match self.get_config_int(Config::DeleteServerAfter) {
+            0 => None,
+            1 => Some(0),
+            x => Some(x as i64),
+        }
+    }
+
     /// Set the given config key.
     /// If `None` is passed as a value the value is cleared and set to the default if there is one.
     pub fn set_config(&self, key: Config, value: Option<&str>) -> crate::sql::Result<()> {