Hide self registration URL if feature is off

`register_url` is only useful if self registration is turned on, so don't mention it unless it is on.

Test Plan:
 * Turn self registration off
 * Go to a course and enable self enrollment
 * Look at self enroll instructions at the bottom of Course Details
 * The self register URL should be hidden
 * The self enrollment checkbox should not mention a code
 * Turn self registration back on
 * The self register URL should be visible
 * The self enrollment checkbox should mention the code

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*NOTE: We have a signed CLA for all of Simon Fraser University (SFU) on file.*