MR widget external status check extension refactor

What does this MR do and why?

Small refactor to the external status checks MR widget extension.

• .then.catch promises to async/await everywhere
• extract mapping code out of component into mappers.js
• fix up tests

This is in preparation for the next MR which will add polling for pending checks, and will need to work with the response object and handle mapping slightly differently.

Screenshots or screen recordings

No functional changes.

How to set up and validate locally

1. Create external status check https://docs.gitlab.com/ee/user/project/merge_requests/status_checks.html#view-the-status-checks-on-a-project (you can use any URL)
2. Create MR for a project
3. External status check widget should display on MR as usual.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #386530 (closed)