[HOTFIX] Several bugfixes and performance improvements

src/models/channel.ts

src/services/impl/channels/add-group-to-channel.ts

src/services/impl/channels/find-all-channels.ts

src/services/impl/channels/find-channel-by-id.ts

src/services/impl/channels/get-channel-stats.ts

src/services/impl/channels/remove-group-from-channel.ts

src/services/impl/channels/remove-user-from-channel.ts

src/services/impl/channels/subscribe-to-channel.ts

src/services/impl/channels/unsubscribe-from-channel.ts

src/services/impl/notifications/find-all-notifications.ts

src/services/impl/notifications/get-by-id.ts

src/services/impl/notifications/send-notification.ts