Add functionality for login and mock signup [feature/signup-login]

:exclamation: This branch is under development and shall only be merged with caution :exclamation:

The intention of this pull request is keep track and discuss a login and signup functionality in the application.
The current vision is to replace the "My Profile"-tab with a "Login"-tab whenever the user is not logged in while the login form itself has a transition to a signup form.
Login will be allowed by email while persisting/remembering the login shall also be possible.

A logout button should most likely be added to the profile-view when its loaded in the "My profile"-tab.