Add a page reporting all system information that might be necessary to debug a GeoServer instance

Create a page, accessible to admins only, that contains all the informations we might like to see to figure out an issue related to the enviroment in which geoserver is running, such as:

• operating system
• jdk distributor and version
• container
• all system, web.xml and enviroment variable

Also have a way to dump the same contents as pure text, for inclusion in a mail or in a jira report.