refactor navbar

Solves #62 (closed), Solves #63 (closed)

Navigation bar refactored, separating the component in different subcomponents. See #62 (closed) for further info.

Also added the name of the logged in user to the navigation bar. See #63 (closed)

Important:

Request to the server side isn't implemented. At the moment only alex@email.de with no matter what password (non empty) is accepted as a valid login.