Make cognate behavior more intuitive #560

Currently, when tag A is a cognate for tag B, it is automatically added to any topic added with tag B. Likewise, when tag B ist removed from the topic, tag A is also removed, but there is no visual indication on the Topic that a certain tag is a cognate for another tag, so users don't really know what tags will be added or removed when they add a tag which has a cognate. This should be made clearer in the UI. Adding a tag should bring along its cognates with some sort of grouping on the UI to indicate that they are joined.