Do not rely on github profile names

[tcibinan]

Problem

Currently, github user profile name is used as a user unique identificator which is completely wrong.

F.e. if a student changes his nickname then he will be classified as a new student. Therefore, two students in the task results table can be a single one in the reality.

Solution

Store not only student and user github profile names but also their ids. All identification should based on a github user id rather than profile name.