improve queue management

- add option to put messages in the queue "on hold", preventing delivery
  attempts until taken off hold again.
- add "hold rules", to automatically mark some/all submitted messages as "on
  hold", e.g. from a specific account or to a specific domain.
- add operation to "fail" a message, causing a DSN to be delivered to the
  sender. previously we could only drop a message from the queue.
- update admin page & add new cli tools for these operations, with new
  filtering rules for selecting the messages to operate on. in the admin
  interface, add filtering and checkboxes to select a set of messages to operate
  on.

webmail/msg.js

@@ -219,6 +219,7 @@ const [dom, style, attr, prop] = (function () {
 		method: (s) => _attr('method', s),
 		autocomplete: (s) => _attr('autocomplete', s),
 		list: (s) => _attr('list', s),
+		form: (s) => _attr('form', s),
 	};
 	const style = (x) => { return { _styles: x }; };
 	const prop = (x) => { return { _props: x }; };