Subscriptions and notifications for namespaces, tasks and lists (#410)

Co-authored-by: kolaente <k@knt.li>
Reviewed-on: https://kolaente.dev/vikunja/frontend/pulls/410
Co-authored-by: konrad <konrad@kola-entertainments.de>
Co-committed-by: konrad <konrad@kola-entertainments.de>

src/services/subscription.js

@@ -0,0 +1,15 @@
+import AbstractService from '@/services/abstractService'
+import SubscriptionModel from '@/models/subscription'
+
+export default class SubscriptionService extends AbstractService {
+	constructor() {
+		super({
+			create: '/subscriptions/{entity}/{entityId}',
+			delete: '/subscriptions/{entity}/{entityId}',
+		})
+	}
+
+	modelFactory(data) {
+		return new SubscriptionModel(data)
+	}
+}