Consider 3 states of sessions

[silvester-pari]

Currently, we have "Open" and "Closed" tabs for the session list. We have received feedback that it's not really clear what that means, and that something like this would be more intuitive:

• Drafts (draft PRs): here it is clear to the user that something needs to be done to submit this
• Under review (open PRs): here it is clear to the user that they are waiting for someone else to do something
• Published (merged PRs) or Done (merged and closed PRs): here it is clear to the user that this is done; not sure if we need to show closed PRs at all, or if its enough to show them on GH;