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PlaceholderAdmin#move_plugin seems to ignore any and all limits placed for the destination placeholder in CMS_PLACEHOLDER_CONF. This means limits can be circumvented by adding new plugins to a non-limited placeholder and then moving them via move_plugin.
The limit-checking code from add_plugin should probably be factored out and used in both methods.
A minor complication is that move_plugin accepts multiple IDs. I'm not sure whether the entire action should fail if the limit would be reached before moving all plugins (providing transactional behaviour) or whether it should try to move as many as possible.
Fixed Moving a plugin to a new placeholder ignores placeholder limits (…
looks great, the move_plugin issue is something for another pull request.
@ojii Everything was fixed but the PlaceholderAdmin frontend which sends the wrong pk due to the js/css hack. So that should be another ticket I believe.