Snapshot channel notes buffers when everyone leaves

Co-authored-by: Mikayla <mikayla@zed.dev>

crates/collab/migrations.sqlite/20221109000000_test_schema.sql

@@ -233,6 +233,7 @@ CREATE TABLE "buffer_snapshots" (
     "buffer_id" INTEGER NOT NULL REFERENCES buffers (id) ON DELETE CASCADE,
     "epoch" INTEGER NOT NULL,
     "text" TEXT NOT NULL,
+    "operation_serialization_version" INTEGER NOT NULL,
     PRIMARY KEY(buffer_id, epoch)
 );