This will allow users to control how their name is displayed in the UI.
A follow-up change will add a host config for changing the default
strategy of choosing the display name. Currently the full name is used
everywhere, which taking a lot of space, so admins will get the chance
with the new config to use first names as the default for display names.
The new account property display_name makes sure that users have full
control over how their names are shown in the UI, e.g. when the first
name default is not the right choice for them.
Change-Id: I62a56ed4323ad10be34f13842018159a79582d0a
9c960cbf72