add customizable tabs functionality

@mattlebel @liuhenry 

Custom tabs FTW. I recreated protos locally by copying the content directly from their page into the ckeditor windows in the admin section. Would love some ideas on how to make this even better. I'm not 100% sure I love the way it figures out (or doesn't) which tab to activate when admins add or remove tabs. Would love to get some thoughts on the best/most intuitive behavior.