When the user adds a category tag in the topic summary of a thread, that thread's Topic page is added to the category. This includes both adding the category tag and adding a template that includes a category tag.
On the Topic page, the category should appear at the bottom of the page, as it does on other namespaces.
In the category list, the thread should appear in the < Topic title on Board name > form that we use in contributions and watchlist.
This involves categorizing Topic pages, and putting a custom interface on the category page so it doesn't just appear as Topic:S842ge5q4ggyxzwf .