Improve Sandstorm usernames management

We now use the `preferredHandle` exposed by Sandstorm as source for
the username and append a number if the username is already taken
since we need to ensure username uniqueness (eg 'max', 'max1', 'max2')

Fixes #352

client/components/sidebar/sidebar.jade

@@ -89,7 +89,7 @@ template(name="addMemberPopup")
         a.name.js-select-member(title="{{profile.name}} ({{username}})")
           +userAvatar(userId=_id esSearch=true)
           span.full-name
-            = profile.name
+            = profile.fullname
             | (<span class="username">{{username}}</span>)
           if isBoardMember
             .quiet ({{_ 'joined'}})