[feature] Push notifications (#3587)

* Update push subscription API model to be Mastodon 4.0 compatible

* Add webpush-go dependency

# Conflicts:
#	go.sum

* Single-row table for storing instance's VAPID key pair

* Generate VAPID key pair during startup

* Add VAPID public key to instance info API

* Return VAPID public key when registering an app

* Store Web Push subscriptions in DB

* Add Web Push sender (similar to email sender)

* Add no-op push senders to most processor tests

* Test Web Push notifications from workers

* Delete Web Push subscriptions when account is deleted

* Implement push subscription API

* Linter fixes

* Update Swagger

* Fix enum to int migration

* Fix GetVAPIDKeyPair

* Create web push subscriptions table with indexes

* Log Web Push server error messages

* Send instance URL as Web Push JWT subject

* Accept any 2xx code as a success

* Fix malformed VAPID sub claim

* Use packed notification flags

* Remove unused date columns

* Add notification type for update notifications

Not used yet

* Make GetVAPIDKeyPair idempotent

and remove PutVAPIDKeyPair

* Post-rebase fixes

* go mod tidy

* Special-case 400 errors other than 408/429

Most client errors should remove the subscription.

* Improve titles, trim body to reasonable length

* Disallow cleartext HTTP for Web Push servers

* Fix lint

* Remove redundant index on unique column

Also removes redundant unique and notnull tags on ID column since these are implied by pk

* Make realsender.go more readable

* Use Tobi's style for wrapping errors

* Restore treating all 5xx codes as temporary problems

* Always load target account settings

* Stub `policy` and `standard`

* webpush.Sender: take type converter as ctor param

* Move webpush.MockSender and noopSender into testrig

internal/workers/workers.go

@@ -54,6 +54,10 @@ type Workers struct {
 	// eg., import tasks, admin tasks.
 	Processing FnWorkerPool
 
+	// WebPush provides a worker pool for
+	// delivering Web Push notifications.
+	WebPush FnWorkerPool
+
 	// prevent pass-by-value.
 	_ nocopy
 }
@@ -90,6 +94,10 @@ func (w *Workers) Start() {
 	n = maxprocs
 	w.Processing.Start(n)
 	log.Infof(nil, "started %d processing workers", n)
+
+	n = maxprocs
+	w.WebPush.Start(n)
+	log.Infof(nil, "started %d Web Push workers", n)
 }
 
 // Stop will stop all of the contained
@@ -113,6 +121,9 @@ func (w *Workers) Stop() {
 
 	w.Processing.Stop()
 	log.Info(nil, "stopped processing workers")
+
+	w.WebPush.Stop()
+	log.Info(nil, "stopped WebPush workers")
 }
 
 // nocopy when embedded will signal linter to