Change topics divider in UI to commas

so we can have multi-word topics
Delete TopicsController#publish

A result of over-enthusiastic copy-pasta
Retrieve all articles on ArticlesController#index

Assume all logged in users can edit all articles for now
(until we differential user and admin).
Extract slug management code into Sluggable concern
Use slugs to access Topics
Create shared example "a model that slugs column", <colname>
Display articles on topics/show