Support loading Preferred first name via mod-user-import

As a library admin
I want to be able to populate Preferred first name via user import
So that I can populate that data from my institution's student information data feed.

Requirement

• Include Preferred first name field in user import schema so that a library can load in the data to populate that field with values.
• Preferred first name is not a required field
• If Preferred first name is not included in the data load using mod-user-import then no change to the field.
  If Preferred first name is included in the data load using mod-user-import then it populates the field and overrides any values that were there before.

Acceptance criteria

Given I want to populate preferred first name values provided by my institution's student data feed
When I use mod-user-import to load in this data
Then Preferred first name field should be an available field so I can upload this information successfully