Add INTERESTS_ENABLED

Adds an option to display the "What are you interested in" options on
the form.
What is the motivation for this?  To my knowledge, this is an option we want for all MAGFest events - or at least for MAGFest, MAGStock, and 8.5-like events.  Was this not turned on for M85?

If this is to support more flexibility for the reg form for other events such as Anthrocon, then I'd suggest a different approach for making forms more customizable rather than adding a config option for every piece of the form we may or may not want to display.