2764: Add wget-friendly Collections file page.

2764: Add wget-friendly Collections file page.

This new route will become the way you share authless Collection links
with others.  They can pass it to `wget -r` to download the whole
collection, nicely organized, with nothing extraneous.  Since it
doesn't try to load user information or look up related Arvados items,
it can be rendered using an API token with a very narrow scope.

Because wget respects robots.txt, this branch stops using that in
favor of the corresponding <meta> tag.  The new view only limits
indexing, so wget can follow the links on the page.

Refs #2764.