Use Page.Params more consistently when adding metadata

[fj]

Right now, .Params is documented as being "everything you put into the frontmatter, except for a long list of specific variables". In particular, values which are otherwise subsumed by Page fields generally don't wind up in .Params. But this has exceptions -- for instance, Description is both a Page field and will be present in Params, even though the documentation says it won't be present in Params.

The net result is that it's a little confusing and inconsistent about what does or does not wind up in Params.

This problem makes it difficult to use certain other Hugo functions in your templates. For example, one cannot do {{ .Param "date" }} because there is no date stored in Params, even though date is in your frontmatter. That means that if you invoke such values dynamically, e.g., {{ .Param $k }}, you can't be sure if it will work, even if $k is the name of a valid key in your frontmatter.

I think a more consistent and less surprising result is that Params should match your frontmatter as closely as possible. This PR attempts to fix this by making sure such assignments always happen.

[bep]

I agree that this is a good change, but it would be good to get the heads up from some others ... @digitalcraftsman @moorereason ?

[fj]

Note: I've updated this branch to remove the conflicts that arose from the Test.Parallel enhancements. If there's anything else I need to do to make this qualify for 0.19 please let me know.

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