Add querystring test URL for fx firstrun. Bugs 901028, 909943.

- Remove old A/B firstrun test URLs

- Remove B firstrun template

- Remove GA experiment code from /firefox/firstrun/

- Add GA experiment code to /firefox/new/
Add tests and improve a little pep8-iness.
Merge pull request #1207 from jpetto/bug-909943-firstrun-test-querystring

Add querystring test URL for fx firstrun. Bug 909943.