Server asynchronous notifications channel

[jordanh]

Issue - Enhancement

We'd like to background some longer running requests, such as #221. The way it's presently written, we get to the end of the process and then return errors.

Errors don't always have to be synchronous. We could let the user know later that a recoverable error happened. JTBD would read like:

• When a user invites their teammates and mistypes and email, they want to know sometime that the email address was incorrect (and be given a link to a process for how to fix it), so that they can onboard their entire team

We could subscribe an outer container to a feed that could pop toasts from the server. That way, long running tasks could be given callbacks which use this channel when things go wrong (or, things go right!)

• Estimated effort: 3 points (see CONTRIBUTING.md)

[jordanh]

No need driving this at the moment.