Fix "Join league" tracking

Unfortunately mixpanel and the `link_to ... method: ` don't work well together, as they both try to override the default click event.

I followed this guy's suggestion to just make it a normal form submit button instead.
http://balazs.kutilovi.cz/2013/05/07/tracking-links-emulating-post-in-ra
ils-with-mixpanel/