Deactivate notifications during import

During a Unified Importer import, comment creation notifications (and potentially others) must be deactivated. A comment created using either:

- `jetpack/v4/import/comments`
- `/wp/v2/comments`

Generate a notification if the post already exists and notifications are on. This will generate multiple unwanted notifications.

Steps to reproduce:

1. If you have 2FA ensure to be logged in https://wordpress.com/me and to have the comments notifications on
2. Create a JN site and export the default xml (it has a default comment)
3. On an empty atomic site, import the xml
4. Import it again

Start by tracing the Unified Importer handling for `jetpack/v4/import/comments` and `/wp/v2/comments`, then reproduce the XML import twice on an empty atomic site with comment notifications enabled. Done means imports no longer generate unwanted comment-creation notifications while the reported existing-post notification behavior remains covered by tests or reproduction steps.

Written by the indexing model from the issue text.