Add diagnostics API endpoint.

Only available with write access and above.

Reports used and available filesystem sizes and database space
usage.

lib/www/server/api/index.js

@@ -312,6 +312,9 @@ app.map({
 			}
 		}
 	},
+	'/diagnostics/': {
+		get: [ mw.auth.access.write, mw.etag.noSave, mw.admin.diagnostics.get ]
+	},
 	'/rss/': {
 		get: [ mw.rss.get ]
 	}