Support parameterization of notification types

[c. b. osborn]

Developers should be able to pass a list of notification types to the Dialect NotificationCenter, to have them rendered on the Settings page.

It might also make sense to use a json schema specifying both the name of the notification, as well as a type. E.g.

[
  {
    "name": "Welcome message on thread creation",
    "type": "Event" 
  },
  {
    "name": "Price changes",
    "type": "Threshold"
  },
  {
    "name": "Deadline reminder",
    "type": "Threshold"
  },
  {
    "name": "Outbid events",
    "type": "Event"
  },
  ...
]

In the above, Event types are discrete event types such as account creation, placed orders, etc. Threshold types are the movement of a scalar past a threshold value.