Compatibility fixes for PHP 8 and YOURLS 1.9.2

[nicwaller]

What

Introduce a few small but essential fixes for compatibility with PHP 8 and YOURLS 1.9.2

• check if constant YOURLS_USER is defined before using it
  • Starting with PHP 8, using undefined constants is now a fatal error
  • This again allows the plugin to be used when YOURLS has public shortening enabled
• check if global domainlimit_exempt_users is an array before using it
  • This avoids a fatal error when $domainlimit_exempt_users is not set in config.php
• remove use of deprecated function yourls_encodeURI()
  • yourls_encodeURI has been deprecated since 1.9.1
• remove use of deprecated function yourls_escape()
  • yourls_escape has been deprecated since 1.7.3
  • Use yourls_sanitize_url_safe instead

This fixes #6

Testing

I added a unit test suite in #11 that gets run with GitHub Actions. The test suite is now passing. ✅

I also completed end-to-end integration testing with these software versions:

• YOURLS/1.9.2
• PHP/8.2
• Apache/2.4.57
• MySQL/8.3

Credit

This combines suggested improvements from:

• Check global is array before reading its values #7
• Replace deprecated yourls_escape call #8
• fix for 1.92 #9