Resolve "login screen - login component logic"

Closes #57 (closed)

Proposed changes

• Error handling: check if input is valid
• Service interaction: interact with service

Testing instructions

Check all possible variations of filling the login form. (both unfilled, both filled, one missing, ...). Click Login

Expected behaviour

• Errors should be shown immediatly, but only if the input field got touched
• When trying to log in, login is always successful. You get redirected to /rooms (maybe not implemented, yet). A message should be shown

If you want to try an unsuccessful login attempt, go to authentication.service.ts and change private mockUser: User = new User(1, 'test', 'test@test.de', true); to private mockUser: User = null;. You should then be redirected to /home and a message should be shown.