feat: Make gossip period configurable (#4346)

This is needed to test periodic re-gossiping in existing chats.

Also add a test for verified groups on that even if "member added" message is missed by a device of
newly added member, after re-gossiping Autocrypt keys to the group it successfully learns these keys
and marks other members as verified.

src/config.rs

@@ -318,6 +318,13 @@ pub enum Config {
 
     /// Last message processed by the bot.
     LastMsgId,
+
+    /// How often to gossip Autocrypt keys in chats with multiple recipients, in seconds. 2 days by
+    /// default.
+    ///
+    /// This is not supposed to be changed by UIs and only used for testing.
+    #[strum(props(default = "172800"))]
+    GossipPeriod,
 }
 
 impl Context {