Remove signin/signup and forgot password from the main site.

Use the dialog instead

issue #900
Updated the tests to do all signing in through the dialog.
Hide the "persona intro" whenenver signing into the Persona main site.
Remove the "Sign in" button from the front page "tour"

issue #3189
Merge pull request #3253 from mozilla/issue_900_main_site_dialog

Issue 900 main site dialog