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
Improve i18n flexibility by repeating full text

This keeps existing translations unchanged, and only introduces new
translation keys.
Fix label[for] across differing translations

The for attribute needs to match the checkbox's id.
Merge pull request #367 from sfu/fix-hide_register_url

Hide self registration URL if feature is off