feat(mr review): resolving and unresolving discussions

This MR follows up on !193 (merged) and connects buttons for resolving and unresolving discussion to actions that call the GraphQL API.

Resolving and unresolving

When the API call fails

Icon selection

@andr3 is concerned that the ❌ button used for "Unresolve" action is going to be confusing people who might expect that X means delete. I think that this misconception might be eased-out by the unresolved discussion showing ✔ icon in the same spot + when we introduce deleting comments, the icon is going to be a rubbish bin.

@pedroms Can you please check the available icons whether you can see some clearer representation of "unresolve" action?

Related to #317 (closed)