_layouts/default.html: Avoid `<body id="">`

...for jekyll pages that don't specify "name" in their metadata.

Note that for pages generated with pandoc (that haven't been migrated to
pure jekyll) we use pandoc to generate an html fragment (excluding html
head etc) with jekyll "front-matter" metadata (including the "name"
field automatically populated from the markdown filename), we commit
this html file to git, and then jekyll generates the complete html page.
So far these are the only actual uses of `page.name` -- none of the
pure-jekyll markdown source files set `page.name`, which is why this
commit is necessary (though not *strictly* necessary, it's only to
avoid the technically-invalid html of `id=""`; perhaps more importantly
it's to avoid confusion or implying that a page *must* set `page.name`).
Landing page: Clearer support for TVs, Consoles, Mobile