Add IRC URL validator and update communication channel format handling

[shreyadesai20042009]

🛠 Changes Made

• Added a custom validator function validate_irc_url to validate IRC channel URLs
• Enforced correct IRC format: irc://<host>[:port]/<channel>
• Made channel part mandatory (no empty or missing channel allowed)
• Added special validation for OFTC URLs:
  • Required format: https://webchat.oftc.net/?channels=#channel
• Updated help text in CommunicationChannel model:
  • Changed [channel] → <channel> (now required)
  • Added OFTC format guidance
• Integrated the custom validator with the existing URLValidator (did not remove existing validation)

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✅ Validation Covers

• ❌ Missing channel (e.g., irc://irc.mozilla.org)
• ❌ Incorrect protocol (e.g., http://freenode.net)
• ❌ Incorrect OFTC format (e.g., irc://irc.oftc.net/qemu)
• ❌ Non-IRC links (e.g., Slack, Discord, etc.)
• ✅ Valid IRC URLs (freenode, mozilla, gnome)
• ✅ Valid OFTC webchat URLs

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🧪 Testing

Tested manually using:

• Django Admin interface
• Django shell using full_clean()

Valid cases:

• irc://irc.freenode.net/#guix
• irc://irc.mozilla.org:6697/webcompat
• https://webchat.oftc.net/?channels=#qemu

Invalid cases:

• http://freenode.net
• irc://irc.mozilla.org
• https://google.com
• irc://irc.oftc.net/qemu

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📌 Notes

• Did not modify any other fields or logic
• Kept existing validation intact and extended it safely
• Ensured compatibility with existing model structure

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🙌 Ready for Review

Please let me know if any changes or additional edge cases should be handled!