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.

lib.ts

@@ -215,6 +215,7 @@ const attr = {
 	method: (s: string) => _attr('method', s),
 	autocomplete: (s: string) => _attr('autocomplete', s),
 	list: (s: string) => _attr('list', s),
+	form: (s: string) => _attr('form', s),
 }
 const style = (x: {[k: string]: string | number}) => { return {_styles: x}}
 const prop = (x: {[k: string]: any}) => { return {_props: x}}