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/src/common/optionsPrototype.json5 b/src/common/optionsPrototype.json5
index a5caa3e..80020ce 100644
--- a/src/common/optionsPrototype.json5
+++ b/src/common/optionsPrototype.json5
@@ -95,13 +95,13 @@
     context: 'options',
     killSwitchType: 'option',
   },
-
-  // Experiments:
   'threadlistavatars': {
     defaultValue: false,
-    context: 'experiments',
-    killSwitchType: 'experiment',
+    context: 'options',
+    killSwitchType: 'option',
   },
+
+  // Experiments:
   'autorefreshlist': {
     defaultValue: false,
     context: 'experiments',