feat: Adds a webhook for when a user follows the stream (#4362)

* feat: Adds a webhook for when a user follow the stream

Frontend:
Adds a checkbox which renders only when social features are enabled.

Backend:
- Defines a new type for the event
- Implements the webhook including unit testing
- Fires the webhook inside the ApproveFollower handler

* Removes unnecessary Type fiels from Events struct

* refactor: renames SendUserFollowedEvent func to FediverseEngagementFollow and fixes linting errors

* fix: fixes unit test to reflect removed Type field from FediverseEngagementFollowEvent

* feat: fires webhook also when manual approvals are not required

* chore: slight cleanup

---------

Co-authored-by: Gabe Kangas <gabek@real-ity.com>

core/webhooks/fediverseEngagement_test.go

@@ -0,0 +1,28 @@
+package webhooks
+
+import (
+	"testing"
+	"time"
+
+	"github.com/owncast/owncast/activitypub/events"
+	"github.com/owncast/owncast/models"
+)
+
+func TestSendFediverseEngagementEventFollow(t *testing.T) {
+	checkPayload(t, models.FediverseEngagementFollow, func() {
+		sendFediverseEngagementEventFollow(events.FediverseEngagementFollowEvent{
+			Event: events.Event{
+				Timestamp: time.Unix(72, 6).UTC(),
+				ID:        "id",
+			},
+			Name:     "be",
+			Username: "be@witch.me",
+		})
+	}, `{
+		"id": "id",
+		"image": "",
+		"name": "be",
+		"timestamp": "1970-01-01T00:01:12.000000006Z",
+		"username": "be@witch.me"
+		}`)
+}