Implement the new welcome to the onboarding screen

New mocks https://projects.invisionapp.com/share/KWU1X9IEB2U#/screens/399136676

• Implement the new design
• Display the username
• Link "No thanks, I'll do it later" to the main activity page
• Link "Lets get setup" to the "Hashtags" profile setup
• e2e tests
• unit tests