Update validate-h-card

[gRegorLove]

This resolves #99 and streamlines a few things on the /validate-h-card page:

• Updated https://github.com/barnabywalters/php-mf-cleaner
• Use the mf-cleaner's updated getRepresentativeHCard method instead of duplicating the rep h-card code inline
• Flatten some of the if/else logic in the page, splitting out if ($error)... and if ($showResult)...
• Use the mf-cleaner's getPlaintextArray to parse multi-value mf2 properties for display
• Updated microformats.org links to https
• Added HTML <label> for accessibility, added required attribute
• Some updates to reduce the amount of PHP notices (when error_reporting is on), using isset and null coalescing operator

You can test these updates on https://iwm.gregoreatworld.com/validate-h-card/

The URL from #99 is working with these updates: https://iwm.gregoreatworld.com/validate-h-card/?url=https%3A%2F%2Fpresence-gqle9t8ex-johanbove.vercel.app%2F

Edit: contextualized the message when there's no rep h-card, but one or more h-cards:

• multiple h-cards: "Multiple h-cards were found on your site! Consider only having one and marking it up..." Example
• one h-card: "An h-card was found on your site! Consider marking it up..." Example

[gRegorLove]

I need to push a small change before this is ready. The second condition (no rep h-card, 1 or more h-cards found) is always saying "Multiple h-cards were found...". I'll make it say "An h-card was found..." if there's only one.

[gRegorLove]

Any possibility we can merge this? I'm not sure how to set up unit tests with this older framework to mock requests. The test links above demonstrate the different states and most of the changes are actually in the mf-cleaner dependency.

[gRegorLove]

I'm feeling pretty confident about this so going to merge with the two approvals. We can work on implementation tests going forward, particularly after #82.