Add optional setting for hiding avatars in <Pill>s

As part of https://github.com/vector-im/riot-web/issues/4640#issuecomment-316659445

src/components/structures/UserSettings.js

@@ -101,6 +101,10 @@ const SETTINGS_LABELS = [
         id: 'MessageComposerInput.autoReplaceEmoji',
         label: 'Automatically replace plain text Emoji',
     },
+    {
+        id :'Pill.shouldHidePillAvatar',
+        label: 'Hide avatars in user and room mentions',
+    }
 /*
     {
         id: 'useFixedWidthFont',