Implement onboarding screens #1
Description
Implement onboarding activity with 3 screens:
Please use as a reference the onboarding section implemented for the old android app: code4romania/monitorizare-vot-android#136
Figma wireframes available here: https://www.figma.com/file/21x1ui3YZJrGnpZNVnEuuQ/DRAFT-MV-mobile?node-id=0%3A1