Incorrect formatting of suggestions imported from GitHub

Problem to solve

If a code change suggestion is made on GitHub and then imported into GitLab, the suggestion does not look like the ones made in GitLab and it is not actionable.

The suggestion is just a box that contains the previous suggestion, but it's not anything that can be applied or used in GitLab. Nor does it provide any indication if it was applied on the GitHub side.

Note: This problem is visible only once this fix is applied. Without this fix, the diff does not even load.

Screenshots

Plan

See #340624 (comment 676314993) for details

1. Change the format of suggestions in markdown when imported from GitHub to match GitLab. !71411 (merged)
2. Create suggestions as DiffNotes instead of the current LegacyDiffNotes on import. !71765 (merged)
3. Add a import duration panel to github importer dashboard https://dashboards.gitlab.net/d/importers-github-importer/importers-github-importer
   • gitlab-com/runbooks!4082 (merged)
4. Rollout the Feature flag #344309 (closed)
   • Enable in Staging
     • Test on Staging
   • Enable in Production (2021-11-08)
     • Test on Production
   • Mark the feature flag as default_enabled: true (!74096 (merged))
   • Remove the feature flag
5. Update the documentation https://docs.gitlab.com/ee/user/project/import/github.html#import-your-project-from-github-to-gitlab