skins: the settings class understands skinName. Send skinName value to the client

skinName must be a single string (no directory separators in it) pointing to an
existing directory under /src/static/skins.
In case these conditions are not met, its value is rewritten to "no-skin".

Also, the value of skinName if sent to the client via clientVars for allowing
its use it in the browser.

src/node/handler/PadMessageHandler.js

@@ -1268,6 +1268,7 @@ function handleClientReady(client, message)
         // client is read only you would open a security hole 1 swedish
         // mile wide...
         var clientVars = {
+          "skinName": settings.skinName,
           "accountPrivs": {
               "maxRevisions": 100
           },