Simple notifications

[notnotdrew]

An idea to send messages to IDE users:

Create an endpoint on learn that returns something like

{ 
  "message": "Hello there",
  "options": {},
  "expires":  "datestring"
}

Where options correspond to the options defined for notifications (replacing button[onDidClick] with something like button[url], and if provided the button just opens the url).

On the client, we check the endpoint maybe every hour. The first time we get a message, we calculate & store a hash of it. Whenever the hash is broken, as long as it hasn't expired, we pop it open.

Alternatively we send an id rather than hashing or something.

[joshrowley]

Maybe not for this PR / release cycle, but I think it'd be also nice to be able to send a type of message that opens up a pop up (the endpoint can have width and height dimensions and then a default) with any URL we want to send students to, that way we have a lot of flexibility.