Improve custom fields checks when changing any setting
I think that some checks need to be done before a important change is done to a custom field. For instance, today I unset the "For all projects" of a custom field, and due to this change some queries which used this field failed (the classic Error 500). In my opinion, when such a change is done, Redmine should check first if the field is already used by some project and, if so, warn the user that the queries may fail, or even best, link the field to the projects that already are using the field.
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