Add ability to create Yubikey second factor

Sorry, being silly.

@rjkip
> Do we want to log something here?

@DRvanR 
> (Disclaimer: did not look at any other code)
> This should not be done in a projector. All data should come from Events. Events are history and known to be the truth, all data they hold is valid (otherwise they may not hold it. There should not be a need to fetch additional data outside of a possible projection that needs to be updated. (Fat Events)

@rjkip
> Ok, will not log. Am I fetching additional data, though? If you mean the Identity entity, are you suggesting we're not projecting a relational model and I should just store second factors with an identity ID?