Add display name field to SIS User Import

The display name (`short_name`) field is **optional**. If absent, `short_name` will behave as before. If present, it will explicitly set `short_name` to the given value, unless it's a stuck SIS field of course.

Previously, if first and last names were not given, the user's name will be a single space. The behavior is the same now, except when `short_name` is present. In that case, an error will be thrown.

This implements this [feature request](https://help.instructure.com/entries/31874660-Add-display-name-field-to-SIS-Import-CSV).

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*NOTE: We have a signed CLA for all of Simon Fraser University (SFU) on file.*