Launch threadListAvatars feature

This CL adds the docs for the feature and promotes the feature from an
experiment to an option.

Fixed: twpowertools:28, twpowertools:26
Change-Id: I69d6eccc5fe83c802718e255fa9967f804f3d9a6
diff --git a/docs/resources/thread_list_avatars.jpg b/docs/resources/thread_list_avatars.jpg
new file mode 100644
index 0000000..85b4f1c
--- /dev/null
+++ b/docs/resources/thread_list_avatars.jpg
Binary files differ