Fix for 144: When loading a plugin from a cross-origin URL, show a dialog warning the user that it could be unsafe

Fix for bug [144] (https://github.com/twosigma/beaker-notebook/issues/144).
In the plugin manager if the user inserts an URL as a source of the plugin, a dialog is shown to ask for confirmation.
At notebook load, if any evaluator is referencing an external URL, a dialog is shown to ask for confirmation. If the user denies the notebook is loaded without the URL-based evaluators.