answering the login request

[bzns]

• receive the login request
• set user status in database to online if request valid
• return the appropriate status code & token (if login successful)

Estimated time: 2h This task is part of user story #4

[Thahit]

https://github.com/sopra-fs21-group-10/td-server/commit/411915346fea496345a96b0dbbe0619f6fd626d4