add createUser method on ApiClient

[ashfame]

This PR adds a method to create the user, which would be authors in our use case. Currently not called from anywhere.

username, email and password are 3 required fields, but only details I can imagine we would have to work with, would be name + possibly a username at times. username is also not modifiable. A DB query is possible of course but it escapes me why it's recommended to not change/edit username.

Regardless this isn't a problem, since we can create temp users with whatever data we have. Afterwards the administrator can create new users with the right details and then just delete the ones created by us. At delete, WordPress prompts with the option of re-assigning all content attributed to the user being deleted to another user. So, it works out.

window.trywp.apiClient.createUser({ "username": "aa", "email": "[email protected]", "password": "aa", "roles":["author"], "firstname": "aaaa", "lastname": "kkkk" })

[ashfame]

@psrpinto & I discussed it would probably be best if user is prompted with author info to be filled out, after we have identified how many authors are there and already processed them. This could be a fitting thing for a "finalise import" step where we would potentially need to run a bunch of things after liberating data to make it fitting for a WP install - such as liberating media assets.