Categorize hackathons as either open to everyone or open only to students

[drryd]

If others feel this would be valuable, I can categorize the existing hackathons and then future submitters can include this information as part of their commit.

[joshualivesay]

+1

[alexmac131]

I actually like this very much, I am tempted at age 50 to enroll just so I can attend the student ones. :)

[japacible]

@djr- Feel free to add the category!