Bring back conversation.leaveGroup for GV1 scenarios

ts/models/conversations.ts

@@ -2793,6 +2793,43 @@ export class ConversationModel extends window.Backbone.Model<
     }
   }
 
+  async leaveGroup(): Promise<void> {
+    const now = Date.now();
+    if (this.get('type') === 'group') {
+      const groupIdentifiers = this.getRecipients();
+      this.set({ left: true });
+      window.Signal.Data.updateConversation(this.attributes);
+
+      const model = new Whisper.Message(({
+        group_update: { left: 'You' },
+        conversationId: this.id,
+        type: 'outgoing',
+        sent_at: now,
+        received_at: now,
+        // TODO: DESKTOP-722
+      } as unknown) as MessageAttributesType);
+
+      const id = await window.Signal.Data.saveMessage(model.attributes, {
+        Message: Whisper.Message,
+      });
+      model.set({ id });
+
+      const message = window.MessageController.register(model.id, model);
+      this.addSingleMessage(message);
+
+      const options = this.getSendOptions();
+      message.send(
+        this.wrapSend(
+          window.textsecure.messaging.leaveGroup(
+            this.id,
+            groupIdentifiers,
+            options
+          )
+        )
+      );
+    }
+  }
+
   async markRead(
     newestUnreadDate: number,
     providedOptions: { readAt?: number; sendReadReceipts: boolean }