templates: one Jinja language (hive-template) for the notify router's flows, flows document version 2

A todo node's key and text were hand-expanded `{field}` strings. They are now Jinja templates rendered through the new hive-template crate (minijinja, an unknown variable is an error), and the controller's save check reads the variables the template uses. The flows document is version 2; a version 1 document reads as version 2 with each `{field}` converted to `{{ field }}`, and the next save writes it.

swarm-notify-router/src/stored.rs

@@ -236,7 +236,7 @@ mod tests {
         );
         for bad in [
             junk(2, b"not json"),
-            junk(3, br#"{"version":2,"flows":[]}"#),
+            junk(3, br#"{"version":3,"flows":[]}"#),
             junk(4, br#"{"flows":[{"id":"x"}]}"#),
             Read::Failed("timeout".to_owned()),
         ] {