added sign in button to individual course page if the user is not logged in (nextAuth will remember the path of where signIn() was called, such that upon successful login, it will redirect the user to the page where the user was)
removed unused and old files
parallell pages and pop-up pages for auth seem a bit needless right now, as the default sign in page provided by nextAuth will do just fine. we can come back to do someting more fancy at a later point
signIn()
was called, such that upon successful login, it will redirect the user to the page where the user was)closes #18