Add INTERESTS_ENABLED

Adds an option to display the "What are you interested in" options on
the form.
This is definitely something Anthro asked for.

An alternative to this would be to have Anthro override the template in their own plugin, but I also don't know if that's a good approach for common config stuff because:

If it's a config option you can turn on and off, then there are less event-specific overrides which means less divergence from the original codebase.  If we make changes upstream, they'll automatically get them, vs if it's an overridden template then there is divergence and less ability to share.

To me, ideally, any event should be able to grab stock ubersystem and customize it to their needs solely by changing up the config options.  So to achieve that goal, I think this is a decent solution.  Eli, do you have alt ideas?