[Go] implement admin pages support in the backend

[sergey-shambir]

Implement REST API for the admin pages The REST API must cover the following cases

• [x] add new contest
• [x] add new assignment to the contest
• [x] add test case for the assignment
• [x] add user (with one of three roles: student, judge, admin)
• [x] add test scenario to the run_backend_tests.py