Consolidate the Param methods

Consolidate the Param methods

Maps in Viper, Hugo's config backing store, is now properly lower-cased not just on top level, the current situation.

While this is mostly a good thing, as you don't need to know the original casing to look up a value, it will be breaking for people doing direct lookups in the ´Site.Params` map.

We will try to find a solution to this "breakage", but the recommended method to get params values is via the `.Param` methods.

This method is now implemented on `Node`, `Page` and `Site` and is case-insensitive:

* Use `.Param "someKey" ` if you want page param with fall back to site param if not found on page.
* Use `.Site.Param "someKey"` to get a site param

See #2590