closes https://github.com/TryGhost/Team/issues/2526
- Mention emails can now be toggled inside staff user' profiles, if they
have the webmention flag enabled on their Ghost site.
- Removed the flag dedicated to webmention email notifications and is
now handled by the `webmention` flag.
- Does not send email notification if `webmention` flag is not enabled.
- Updated tests.
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Co-authored-by: Fabien "egg" O'Carroll <fabien@allou.is>
ce567b9816