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.

This is a follow-up of https://github.com/instructure/canvas-lms/pull/438, which added `short_name`.

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*NOTE: We have a signed CLA for all of Simon Fraser University (SFU) on file.*