LPS-43009 Missing commit: use the right context for plugins and portal resources

Hey Brian,

I'm afraid I missed these changes. They were included in my very first pull-request for LPS-43009 (see epgarcia/liferay-plugins/commit/34b7eb43888a2dd56b326953d12e1a57bbf51398) but I guess that they got lost during the re-sending and re-basing. I noticed it while preparing a complete demo of the RTL hook.

Basically, we need to use the portal context to serve the portal CSS resources that have been overwritten by the hook, and use the plugin context to serve the theme CSS resources. Thus, we now have two filters, one for plugins (DynamicCSSFilter) and one for portal resources (PortalDynamicCSSFilter). The latest extends the first and simply initializes itself with the appropriate context.

Thank you very much