Fixes Moving a plugin to a new placeholder ignores CMS_PLACEHOLDER_CONF limits

This is a patch for issue #1646.

Basically I modified the Javascript to display a message when a user attempts to move or add a plugin either in the front or backend and the action was not successful. I also modified the move_plugin to check for the placeholder limits and moved the checking functionality to a function (```has_reached_plugin_limit```) in separate module, which raises an exception (```PluginLimitReached```) . Also created a new test case for ```PageAdmin``` methods which allows us to test methods like add_plugin and move_plugin. And last but not least I fixed  a ```NameError``` occurring under certain conditions in ```testcases.py```