Update meta tag generator to link tag generator

[huijing]

Closes https://github.com/WICG/webmonetization/issues/315

This PR addresses the issues mentioned in #315:

• Rename "Meta Tag Generator" to "Link Tag Generator"
• Remove the option to "Use receipt verifier service". Receipts are deprecated in the new spec.
• Change output to use instead of
• 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 $

🎩 Top-hatting instructions (i.e. testing before merge)

1. Pull down this branch
2. Navigate to http://localhost:3000/meta-tag
3. Check that the content updates are correct ("Meta" to "Link")
4. Check the input:
   • Enter a proper URL like https://www.google.com and click "Generate", the generated code should have href="https://www.google.com"
   • Enter a shortened URL like $cows.dev and click "Generate", the generated code should have href="https://cow.dev"
   • Enter a rubbish string like fdjsaklfjdsklfdjas and click "Generate", the error message should trigger and the generated code will not update