Refactor Meta Generator to use Link and re-brand it as the Link Generator

[AlexLakatos]

Currently, https://webmonetization.org/meta-tag helps you generate a meta tag with a payment pointer. That's designed for the old Web Monetization specification.

We should update the page & generator functionality to conform with the new Web Monetization spec, that uses link tags instead of meta tags. https://webmonetization-preview.netlify.app/specification.html

• [x] Rename "Meta Tag Generator" to "Link Tag Generator"
• [x] Remove the option to "Use receipt verifier service". Receipts are deprecated in the new spec.
• [x] Change output to use <link rel="monetization" href="https://example.com/payment-pointer"> instead of <meta name="monetization" content="$example.com/payment-pointer" />
• [x] Add functionality to convert/support both payment pointer notations in the input ($ilp.dev and https://ilp.dev are equivalent). Both forms of payment pointers should generate the link with https, not with $

Note: link tags are supported throughout the document, not just in the head.

[huijing]

Dev notes

1. Should we rename the slug to link-tag and add a redirect from /meta-tag to /link-tag?
2. Since receipts are deprecated in the new spec, we should update the documentation (http://localhost:3000/docs/receipt-verifier/#use-our-publicly-available-receipt-verifier) accordingly to indicate as such.
3. We are sure we want to use https://example.com/ in the generated URL?
4. What's with the margin-top: 50px between the 2 paragraphs of text 🤨?
5. Do we need to cater for the special case for URLs with no path? (https://docs.openpayments.guide/docs/payment-pointers#shortened-form)