Add full & sortable name fields to SIS User Import

Both fields are optional. If absent, the user's name and sortable_name
will be set in the same manner as before.

The full_name field takes precedence over first_name and last_name. The
sortable_name field takes precedence over the inferred value.
Merge pull request #445 from sfu/feature-sis-user-names

Add full & sortable name fields to SIS User Import

Change-Id: Ib8617572802ca69913b422d22301299ab75c211f
Reviewed-on: https://gerrit.instructure.com/34548
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>