Use invite system to invite course participants to topic in secure category

Hi Gassim,

This is an interesting way to use the invite system. I changed the topic title to reflect the contents, to hopefully attract more people here who might be trying to do the same thing.

@dan will take a look at this. I agree we should not have a max number of redemptions allowed. Though the default should still be 1.

This is a bug @dan is working on too. Stay tuned over in that topic.