Customize Updates

All the changes for customize.

* main Customize page (covers the customize panel)
* update the embedding page (covers inline and popover embeds)
* email marketing page (updated workflow and screenshots)
* re-directing embed-screen to customize (death to the SuperEmbed, long live the SuperEmbed!)

Not yet done, but ready to start reviewing @MaxPower15 for content and @jordanmunson for copy.