[iOS] Enrolment button handling in non-logged-in experience

We need to implement the functionality to handle enrolments for non-logged-in users. When they tap the enroll button on a course about page, they should be directed to the registration screen. Ideally, upon successful login or registration, the course should be enrolled for the user and they should be directed to the course home screen.

Acceptance criteria:

• Tapping the enroll button should lead the user to the logistration screen.
• On successfully logging in or registering the user should return to the course about page for the native experience.
• There needs to be sticky bottom banner having the register and sign in buttons a they explore discovery without logging in.