Allow plugging explorer webengine pages under root

The plugin extension point allows adding new webengine pages under a given distribution.

The goal here is to allow plugging pages right under the root, to show generic pages that are common to all distributions, like pages showing custom statistics.

Since the distribution name is usually under the root in the URL, let's use the same prefix than for the default statistics page located at `/nuxeo/site/distribution/_viewstats`: only pages starting with "_view" will redirect to custom webengine pages.