fix(Prejoin): Make prejoin avatar match the other avatars

css/premeeting/_premeeting-screens.scss

     .avatar {
         text {
             fill: white;
-            font-size: 26px;
-            font-weight: 400;
         }
     }
 

react/features/base/premeeting/components/web/Preview.js

                     <Avatar
                         className = 'premeeting-screen-avatar'
                         displayName = { name }
-                        dynamicColor = { false }
                         participantId = { _participantId }
-                        size = { 180 } />
+                        size = { 200 } />
                 )}
         </div>
     );