assignToCurrentChannel publishes ChangeChannelEvent prematurely

[DanielBiegler]

Describe the bug

The following function inside the ChannelService:

    /**
     * @description
     * Assigns a ChannelAware entity to the default Channel as well as any channel
     * specified in the RequestContext.
     */
    async assignToCurrentChannel<T extends ChannelAware & VendureEntity>(
        entity: T,
        ctx: RequestContext,
    ): Promise<T> {
        const defaultChannel = await this.getDefaultChannel(ctx);
        const channelIds = unique([ctx.channelId, defaultChannel.id]);
        entity.channels = channelIds.map(id => ({ id })) as any;
        await this.eventBus.publish(new ChangeChannelEvent(ctx, entity, [ctx.channelId], 'assigned'));
        return entity;
    }

The function doesnt actually persist anything to the DB, it only patches the passed in object which you have to save separately. So if something goes wrong before or when saving your entity, you still publish the event which can lead to bugs if you have logic thats dependent on the ChangeChannelEvent as now your entity has in fact, not been assigned to a new channel. In the event itself the entity will have .channels assigned though, so everything will look proper but be wrong.

To Reproduce

1. Subscribe to the ChangeChannelEvent
2. Call assignToCurrentChannel on some entity but then fail to persist it
3. Now your subscriber got called with a wrong event

Expected behavior

The event should only be published after actually persisting to the db. This will require a refactor of many services but keep in mind that plugins that involuntarily relied on this will be affected too. ☹️