DAR-1730 Performance changes connected to Follow extension

What has been done here:
* re-used `$data` variable when passing it to `EasyTemplate` -- one less call to `FollowModel::getWatchList`,
* re-factored `FollowModel`: changed `intval()` calls into typecasting and extracted (what could be extracted) from inside the loop, moved namespaces array to static field, split `FollowModel::getWatchList` into three methods,
* updated PHPDoc.