IndieWebify h-entry validator should check rel=canonical links

[tantek]

IndieWebify’s h-entry validator should check for the presence of any rel=canonical links, and at a minimum verify that their href attribute is a valid URL, and provide a link to check that URL for a valid h-entry as well.

More enhancements are possible, however even this kind of minimum syntax check, and prompting for subsequent h-entry validation would help catch common typo errors.

Separately, if a rel=canonical link is found, and no u-url is found, then see #27 for the suggestion to use u-url instead or at least first, before worrying about rel=canonical.

(Originally published at: https://tantek.com/2020/069/b1/)