Add INTERESTS_ENABLED

Adds an option to display the "What are you interested in" options on
the form.
I was going to say the opposite; I'd much prefer template overrides to making everything configurable.  There are 12 rows on our current prereg form where people enter data, and the only ones that I can't imagine any event doing without are Name and Email.  So I don't think that it's a good idea to go the route of making up to 10 different fields configurable.

If we DID go the route of making this configurable, we'll also need to change all of the other forms that have this field, including:
* the form that lets group members claim a badge
* the form that lets people claim a placeholder registration or update their info
* the admin form where interests are also displayed

These forms are all different enough that there hasn't yet been any attempt made to unify them such that we can decide what fields are displayed everywhere by editing a single place.  As this kind of feature becomes more common, it might make sense to put in the work to make a single "attendee info" template which is included in all of these places so that someone like Anthrocon which wants to customize the reg form can just override a single template.

In general though, a lot of these fields have backend logic as well.  Like if someone wanted to get rid of the zip code being required (or even asked for), that requires a change to the backend validation logic as well.  So even if it turns out that Anthrocon specifically only cares about eliminating some of our 6 optional rows on the reg forms, that's not necessarily going to be the case for all events, so in the long run we'll need to make validation more pluggable.

In the meantime, I'd prefer template overriding, since that keeps complexity out of the main codebase.   (On the other hand, if this really is the only field that Anthrocon needs to get rid of, I'm not super-opposed to making just this one field configurable just to make their lives easier, just this once.)