For CMS/platform, default to a different external properties file than for sites

When loading HST properties from an external file, it is useful to separate the properties that are used by the CMS/platform instance of HST from the properties that are used for sites. This allows, for example, to prevent easy access from component code to external service references and credentials that are only needed for the CMS.

I propose setting a new default of "${catalina.base}/conf/platform.properties", as opposed to "${catalina.base}/conf/hst.properties" used by sites. This can still be overridden by project developers in the web.xml using the same context property or servlet filter property as currently documented. Values from system properties will still take precedence over all file-based config, as currently documented.