Support for push notifications

[amcclain]

Tracking ticket for potential support in Hoist for "native" style notifications on desktop and potentially mobile.

See https://web.dev/explore/notifications for a nice overview of this entire system / ecosystem.

Note that we could split this support into two categories or phases:

• Purely "browser local" notifications triggered by a running site (or service worker, although those are not something we currently have in play with Hoist).
  • Perhaps pretty limited in its utility, especially given changes to how aggressively both browsers and Hoist throttle inactive tabs, but significantly simpler.
• Server-side originating notifications via the Web Push API, triggered by an HTTP request from our server to a browser-specific channel, without requiring the user to have the site loaded and active in their browser.
  • Would require a server-side component to store per-user subscriptions, handling of keys, and agreement from clients about what kinds of content would be suitable to send in this way.