ANTARCTICLE-247 Case-insensitive authentication in Antarcticle

Fixed Test Scenario:
      1) Log in to Antarticle as PETYA
      2) Click on username and select "My articles"
      Actual result: another user's (petya) data is displayed
      Expected result: PETYA's data is displayed

Solution: Added case-sensitive filtration by username in DAO layer (UserRepositoryComponent)