Backport fix for space watcher behaviour to 5.9.2

It would be good to backport the fix for CONF-36503 so that BTF customers don't have to wait until Confluence 6.

NB: This version of the fix does not include the upgrade task to automatically remove any current space watchers that should no longer exist. However, if you remove anyone from a space as of 5.9.2, all space watchers for that space are double checked, and removed if necessary.
So, if you have any space watchers that you'd like to remove from a particular space, you can re-add one of those users to the space and then remove them again. That will check and remove any watchers for that space that should no longer be watchers.

All space watchers that should no longer exist will be removed as part of the upgrade task that will run in Confluence 6.