Decouple inbox from PR approval state

Currently, the only way to remove a pull request from my inbox is to approve it. This is a usability/productivity problem when I have many pull requests in my inbox, some of which I've already reviewed, but not approved, and others that I've not yet reviewed at all. It can be difficult to find which PRs I have not yet seen.

Other issues (e.g. STASH-2999, STASH-4633) have asked for multiple PR statuses (e.g. complete, disapprove, pending changes). While I support those suggestions, I also see the complexity in creating a design that satisfies many different workflows. I believe that simply decoupling inbox state (in, out) from PR state (not approved, approved), would go a long way to making PRs more manageable, and pose no barrier to more complex solutions in the future.

Regarding design: Imagine an 'x' next to each PR in your inbox. Clicking an 'x' would remove the PR from the inbox (or alternatively gray out the entry). It would return to the inbox when a new commit is pushed. Otherwise, system behavior would remain unchanged (e.g. email alerts with other reviewers' comments would still be sent).

Thanks for a great product. I believe this suggested enhancement is in the spirit of the product's current feature richness delivered through a parsimonious design. I hope you agree.