Thread view needs some improvement

The threading display is a bit confusing with quoted text. The color bars on the left are too subtle to be used for real demarcation of quoted portions, and don't work at all for people with difficulty discerning color on their screen.

With a complex thread with multiple replies, the order can be quite confusing as well.

Compare [This lists.apache.org thread](https://lists.apache.org/thread.html/b478645815bf7b1fee8342a21924162decc173d91c22eaf5f708a435@%3Cdev.fluo.apache.org%3E) and [This Fedora (hyperkitty) thread](https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/thread/MU5IJ2JR6OFMSV6CPHUV7NNKPQPH4GCA/).

In the latter, the indentation makes things much easier to follow, and the quoted portions are subdued a bit better. Though the latter isn't perfect, it does do a slightly better job of threading, and removing duplicate information from each response (like the list address).

Perhaps PonyMail's threading could be improved by adopting more indentation to show the threading in a more layout-driven way rather than color-driven way.

PonyMail works best when there are no inline responses, but email list threads do include many inline responses, and broken quoted portions, interspersed throughout the conversation.

Start by comparing PonyMail's thread view with the linked lists.apache.org and Fedora Hyperkitty examples, focusing on quoted text, reply order, indentation, and duplicate information. Done should mean that complex threads and inline or broken quoted responses are easier to follow without relying on color alone.

Written by the indexing model from the issue text.