Revised Tours for Firefox Test Pilot

Revising the mini-experiment tours for Firefox Test Pilot. The current experience has a few issues:

- There is a button acting as a gateway to enter the tour. This is an unnecessary extra step.
- Once the tour has started, there is no way for the user to exit it until they go through all the slides.
- There is no indicator of progress so the user doesn't know how much of the tour is left or where they are in it.
- On the last slide there was no way for the user to return to previous slides.

The revised tour corrects these issues.