Add discussion item component

I'm not too sure what the right name for this will be, but discussion-item is a good enough working title for now.

I need to add this for two of my deliverables this month, so I thought it best if I make it a UI component.

It's very similar to discussion items used in several places in GitLab so it makes sense to align them. There's not currently anything in Pyjamas to aid the spec, but it would make sense to add it.

Here are some more instances of where this component is already used in one form or another:

Discussion on an issue

Vulnerabilities on the group security dashboard

There's probably a few more examples, but you get the idea.