Announce first blood instead of every flag attempt and solve

[xusheng6]

Summary

• Stop Discord messages for every flag attempt and routine solve, while keeping flag validation audit records in the database.
• Announce only the first recorded solve of each challenge in the public Discord channel.
• Send the challenge author one in-site notification when their challenge is first solved.
• Keep the solver's on-page success message without creating an in-site solve notification for them.

Validation

• .venv/bin/pytest -q tests/test_solves.py tests/test_services_and_failures.py --disable-warnings (82 passed)
• git diff --check