21998: properly percent-encode paths in keep-web directory listing page

21998: properly percent-encode paths in keep-web directory listing page

Use percent-encoded form of the relative URLs as the value of "href"
attribute in directory listing page generated by keep-web

The "wget" command example shown on the page now has single-quotes
around the URL argument.

For testing involving complex URL or path patterns, use the HTML parser
provided by golang.org/x/net/html to scrape the directory listing page,
instead of using regular expressions that may get unwieldy.

Minor edits to the HTML template for compliance and ease of testing.

Arvados-DCO-1.1-Signed-off-by: Zoë Ma <zoe.ma@curii.com>