Pass a URL as a query parameter to the surveys

We need to be able to display destination URLs in the questions. Here's how it works:
1. The background script gets the destination URL as part of the event
2. Background script sanitizes the URL
3. Background script appends the sanitized URL as a query parameter when opening survey.html
4. The survey driver checks that the URL is sanitized
5. The URL can be used in questions (as part of textContent)

Extensions do not have questions about URLs, so this only applies to the non-extension surveys.
This CL also adds test cases for the sanitization method.
See my comment for background.js, I feel like you can do away with the sanitizer.