Ghostery Dawn Intro Hub Miscellaneous Fixes

[benstrumeyer]

• Have you followed the guidelines in CONTRIBUTING.md?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you added an explanation of what your changes do?
• Does your submission pass tests?
• Did you lint your code prior to submission?

• Add back button to each page except Welcome Screen
• If the user wants to return to Step 1 - Sign In via the Step Progress Bar or Step 2 back button, they will be logged out
• Adds back translation string, already in GH-2218
• Prevents user from using the StepProgressBar to move forwards, only back
• Every screen should be responsive except Step 3 - Choose Your Search