switch between the sandbox and real amt

Though I cannot access to the real amt, I have tested that the switch worked when I want to use the sandbox. 
Well, the base method was already defined as an instance method before I made these changes. I was also surprised actually when I saw it. But it also worked because 'interface.get_frontend_url()' called the static method in the amt subclass. I will change to static methods both of them as well as 'get_backend_url' and 'get_assignment_url' in basecrowd/interface.py.