Make implied directed edges explicit

[vincerubinetti]

We talked about this a while ago. I feel that edges that can only be in one direction should be explicitly shown to be directional, like compound -> side effect:

Maybe there are database reasons why the direction property should be left "neutral"/undirected for these types of edges, but at least for the front end, I don't see why they shouldn't be shown as arrows.

[dhimmel]

I am starting to come around on this issue. We only use directed metaedges when the source and target metanode are the same, like Gene→regulates→Gene. However, users expect direction when thinking about the relationship between metanodes of different types. I'll have to do a bit of a deep dive into the hetio codebase and past records on this topic to make sure I'm accounting for all of the design decisions.