Empty username property

I am pretty sure that everyone entered a username at signup, but when querying with user show, there is no entry. There is also no way of adding it using user update.

This is rather unhandy for some services, such as Vikunja which I am adding now, which allows mentioning users by username.

And while we are at it, I would like to predefine the username for my users - since we all use company-emails on stackspin, it can simply be the name-part.