IndieWebify h-entry validator should check rel=canonical links

IndieWebify’s [h-entry validator](https://indiewebify.me/validate-h-entry/) 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](https://github.com/indieweb/indiewebify-me/issues/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/)

Start at the h-entry validator linked in the issue and review how it currently checks entry properties. Confirm that rel=canonical links are detected, their href values are checked as URLs, and a follow-up link is provided for validating the canonical URL; also review issue #27 for the u-url relationship.

Written by the indexing model from the issue text.