Is there a way to allow users to control alignment of images in the Redactor plugin using Craft 3
It looks like the right way to do this is to tweak the redactor settings:

https://github.com/craftcms/redactor#redactor-configs

and 

https://imperavi.com/redactor/docs/settings/image/#s-imageposition

All you have to do is add this key to your redactor config:

"imagePosition": true,


and clear your control panel cache.

The option for image position shows up on the 'edit image' screen.

If you want to use classes to control your image alignment and you want the redactor fields to continue to be WYSIWYG, you need to share those CSS classes between the CP and your front end. This answer will probably help: Where do I add custom css for the Redactor editor in the Craft control panel?