Mark up the avatar initials so they're not read out by screen readers

src/skins/vector/css/atoms/MemberAvatar.css

@@ -23,8 +23,9 @@ limitations under the License.
     position: absolute;
     color: #fff;
     text-align: center;
+    speak: none;
 }
 
 .mx_MemberAvatar_image {
     border-radius: 20px;    
-}
+}