ask users to confirm before irreversible actions

certain actions on the site, such as leaving a team or canceling an
event, should have a warning dialogue making sure the user is absolutely
ok with this action. for now we'll just use javascript's confirm pop-up.
use javascript tests!

my test coverage is pretty good so far for models but for everything
else it's abysmal. I will not let my javascript get out of hand in the
same way.