refs https://github.com/TryGhost/Team/issues/1666
- it seems like we may have a situation where `.activateTheme()` can be called simultaneously resulting in unexpected behaviour in the sync such as duplicate theme setting records
- adjusted behaviour to keep track of the currently running activation within the service and if `.activateTheme()` is called again whilst it's in progress it will wait for completion of the first sync before exiting early or continuing with a new activation
**Note:** There is a known edge-case if there are _more_ than 2 parallel `.activateTheme()` calls. We don't believe that will be an issue but calling it out in case we do still see duplicated custom setting records being created.
Co-authored-by: Kevin Ansfield <kevin@lookingsideways.co.uk>
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