Don't show blockquote indicators on both sides for 6th+ level

We shouldn't show the colored vertical lines and blockquote indentation on both sides of a quotation, only one side (the left side for most people, and I suppose the right side for those who use a right-to-left writing system locale).

Otherwise, the problem with long top-posting email exchanges is that you end up with this (also, performance deteriorates noticeably):

I can't avoid this reality because the majority of the world in the workplace (or business/clients/suppliers) use Outlook and top-post, and most threads need to retain the top-posting as a "paper trail" especially in support or legal scenarios.