Various ARIA a11y fixes.

Notate RightPanel tabs.
Shorten Screen Reader queues.
Make AccessibleTooltipButton screen reader friendly
Flatten DOM for Sticker button using React Fragments

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

src/components/views/groups/GroupMemberList.js

@@ -222,7 +222,7 @@ export default createReactClass({
         }
 
         return (
-            <div className="mx_MemberList">
+            <div className="mx_MemberList" role="tabpanel">
                 { inviteButton }
                 <GeminiScrollbarWrapper autoshow={true}>
                     { joined }