💻Change where the buttons on the front page send you, remove strange preview mode

[MarleenGilsing]

For the hackathon

Right now, the 'start learning' button sends you to Hedy level 1, which is great. We only want to change the name of the button to 'Let's get started!'. The 'start teaching' button sends you into a very unexpected preview mode, that nobidy is using by choice. We'd like to remove this preview mode, to change the button to 'Create an account' and send the user to the signup page.