feat(mr review): display whether discussion is resolved or not

This is the first out of two MR's to implement resolving diff discussions.

It fetches resolved and resolvable flags for each discussion and shows a checkorcross` button representing the action to resolve or unresolve discussion.

resolved	unresolved
shows a button to unresolve	shows a button to resolve

The buttons don't trigger any action. That's going to be implemented in the next MR.

How to review

This MR can be easier reviewed as two distinct commits.

Action vs. State

I admit that the showing ❌ as an indication of resolved MR is a bit tricky, but we do it so the button represents the action of unresolving. At least there is a hover message explaining the action 🤷 I wasn't able to come up with a better solution. All available icons can be found here: https://microsoft.github.io/vscode-codicons/dist/codicon.html

Related to #317 (closed)