Fixed 1,2,7

[AndreiUkladchikov]

1. Добавил закрытие монго в shutdown
2. Поправил импорты в ugc_backend по pep8: Imports should be grouped in the following order:

Standard library imports. Related third party imports. Local application/library specific imports. You should put a blank line between each group of imports.

7. В тестах используются HTTPStatuses