Add codelab redirect page.
Add codelab redirect page.

Bug: 28767461
Change-Id: I0dd40da47df192c4734dfaffd343fff464dabbd1

samtstern/FirebaseUI-Android,samtstern/FirebaseUI-Android,SUPERCILEX/FirebaseUI-Android,firebase/FirebaseUI-Android,firebase/FirebaseUI-Android,SUPERCILEX/FirebaseUI-Android,JosefHruska/FirebaseUI-Android,firebase/FirebaseUI-Android,MaciejCiemiega/FirebaseUI-Android,SUPERCILEX/FirebaseUI-Android,JosefHruska/FirebaseUI-Android,samtstern/FirebaseUI-Android,firebase/FirebaseUI-Android,SUPERCILEX/FirebaseUI-Android