Add the CategoryModel::filterCategoryPermissions() method.

This method allows you to restrict an array of category IDs to just the categories that the current user can use.
Fix the doc block for CategoryModel::Categories().
Fix some incorrect variable typos in the DiscussionModel.
Optimize DiscussionModel->GetCount() slightly.

Iterate over only the categories that are being asked for rather than all categories. Note that there is some commented out code to aid in testing the old method against the new method.
Make DiscussionModel->GetAnnouncements() support an array of category IDs in the where rather than just one.

An array of category IDs should behave like a filtered recent discussions list.
Add DiscussionsController->setCategoryIDs() and DiscussionsController->getCategoryIDs() to allow plugins to override the categories being viewed in recent discussions.
Merge pull request #2237 from vanilla/feature/discussions-category-filter

Allow recent discussions to be filtered by an array of categories.