validate organisation selection against valid options

[levgorbunov1]

• Validates user selection for organisations against valid options set in configuration.

[github-actions]

🦙 MegaLinter status: ❌ ERROR

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ PYTHON	black	2	2	0	0.85s
❌ PYTHON	flake8	2		6	0.46s
✅ PYTHON	isort	2	2	0	0.17s
❌ PYTHON	pylint	2		3	4.94s
✅ REPOSITORY	gitleaks	yes		no	0.46s
✅ REPOSITORY	trivy	yes		no	5.21s

See detailed report in MegaLinter reports
Set VALIDATE_ALL_CODEBASE: true in mega-linter.yml to validate all sources, not only the diff

MegaLinter is graciously provided by

[connormaglynn]

Also worth doing the same validation in the github_service class as an extra measure to ensure that the value are valid before actually using them to send the invitation 📬

Happy for that to be a separate PR though 😁

[tamsinforbes]

🌝🌖🚀LGTM🚀🌘🌚

Minor comment to update the logging wording to match the changes in my commit.

[tamsinforbes]

🌝🌖🚀LGTM🚀🌘🌚