Reading time added to Articles API

https://github.com/forem/forem/issues/13436 brought up the need to have the reading time shown in the api response for articles response.

This felt like a 15 minute fix (and a good chance to learn more about our api code), which delivered immediate value to someone, so I added it. You should see a new value "reading_time_minutes" in the /articles, /articles/:id, and /articles/me responses, which is an integer number of minutes, which matches what's shown in the web UI for "7 minute read"