Updates to the newsletter subscriber form UI

Updated the default sample form that Audience generates for the newsletter, it now has some inline CSS styles. So you can just copy and paste and it works.

I've cleaned up the UI around the subscriber forms to bring them into line with the newsletter UI. Put a new menu in place.

I added the Newsletter logo to the newsletter name on the top of the screen and the menu when you are looking at that newsletter.

Added a new "Build your own Form" section that has all the details you need to create form and tie it to the Audience API