Bug 808576 - Revise layout of New Mail Account

r? @asutherland

This PR changes the style of the mail setup screens to use the "organic" skin, and also fixes bug 834451. Finally, I cleaned up the FormNavigation code so that things are a bit more encapsulated. The separate initialize() function made it seem like you could call that multiple times, and you couldn't, so I just removed it.

https://bugzilla.mozilla.org/show_bug.cgi?id=808576
https://bugzilla.mozilla.org/show_bug.cgi?id=834451