fixes https://github.com/TryGhost/Team/issues/1900
refs https://github.com/TryGhost/Team/issues/1901
- Defaults to the same value as the current email_track_opens setting for existing installations, otherwise defaults to true
- Had to use a custom migration because the `addSetting` helper doesn't support using an existing setting as current value
- Added a minimal UI to change the setting, but this still needs some design magic 🪄✨
- Link replacement is disabled if `email_track_clicks` is disabled. In the future we might consider to still do parial additions, such as source attribution and maybe redirects (to discuss).