Extracting CategorySelect.class.php from CategorySelect extension

There are few extensions using helper methods from <code>CategorySelect</code> class. In each place when it's being used it's also wrapped with if statement. After short discussion we decided to extract this class:
* moved the class to <code>/includes/wikia/helpers/</code> and renamed it to <code>CategoryHelper</code>,
* removed the line in setup file and add one more lines to <code>DefaultSettings.php</code>,
* unit tests were also moved and renamed,
* created <code>CategorySelectHelper</code> class in the <code>CategorySelect</code> and left there two helper methods which are connected only to the extension,
* added more PHPDoc and did small adjustments after linting.

@federico-lox @owend @kflorence feel free to left me notes :)