Global reactivity: Connect status of issuable with a global state reused in Notes app

Proposal

Currently the status of the issuable in MRs is not connected to the state in the Notes causing the following flow:

1. User merges MR in the MR widget
2. The comment box at the bottom of the Notes app still displays: "Close merge request"

Kapture_2021-04-21_at_13.18.20

Context

This is a first practical application of the bigger topic of Global Reactivity, being a real life application of the concept explored in the proof of concept: !51719 (merged)