Make sure that website managed beans injected into platform managed objects always execute with website class loader

HST site webapps can inject customization which are used in the HstModelImpl for the website. However, the HstModelImpl is managed by the platform webapp.

Since the website managed beans that get injected might require the specific website class loader instead of the current thread's classloader, we need to make sure that the website classloader is used when those objects' methods get invoked.

For example, when an HST site webapp inject custom ChannelFilter objects, this code is executed typically by a CMS webapp thread (and this cms class loader). Hence we need to switch classloader during those method invocations

Likewise for example for the LinkCreator customizations: For example when we want to resolve cross webapp links, the customizations of, say, the LinkProcessor, per webapp should be executed with the webapp its class loader