* Improves tests for webpush notifications
* Sets subscriber correctly in web push payload (without this,
notifications do not work in Safari)
* NOTE: for now, I'm using the Coder Access URL. Some push messaging
service don't like it when you use a non-HTTPS URL, so dropping a warn
log about this.
* Adds a service worker and context for push notifications
* Adds a button beside "Inbox" to enable / disable push notifications
Notes:
* ✅ Tested in in Firefox and Safari, and Chrome.
* Adds `codersdk.ExperimentWebPush` (`web-push`)
* Adds a `coderd/webpush` package that allows sending native push
notifications via `github.com/SherClockHolmes/webpush-go`
* Adds database tables to store push notification subscriptions.
* Adds an API endpoint that allows users to subscribe/unsubscribe, and
send a test notification (404 without experiment, excluded from API docs)
* Adds server CLI command to regenerate VAPID keys (note: regenerating
the VAPID keypair requires deleting all existing subscriptions)
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Co-authored-by: Kyle Carberry <kyle@carberry.com>
