Convert tags to smaller set of blog post categories

In the process of migrating the Drupal site to Lektor, @duncan and I agreed it might make sense to replace the blog post tags with a predefined set of categories. This would simplify the Lektor site structure since we'd be able to forgo the use of the lektor-tags module and allow us to provide RSS/Atom feeds of posts in each category.

The list of tags currently used on the blog is available here: https://pad.riseup.net/p/QqaP7feFvQlPJrDHmSyX