Modified SQL for "TotalTopics"

#### Update for [OUR-24 - Excessive pagination](http://issues.umbraco.org/issue/OUR-24)

See comment: [#67-4298](http://issues.umbraco.org/issue/OUR-24#comment=67-4298) - modified the SQL for ```TotalTopics``` to perform a ```COUNT()``` on the topics rather than a ```SUM()``` on the forum's topics.