code monitoring: add url validation for slack and webhooks

Created by: limitedmage

Part of #27163 (closed)

• Slack webhook URLs are now required to start with https://hooks.slack.com/services/
• Generic webhook URLs are required to start with http:// or https://
• Updated test error messages to small font to match form validation error messages

Security Note

This is only meant as a convenience to benevolent users. If URL validation is needed for security reasons, it should be done on the server. See Slack conversation about security implications.

Screenshots

Populated, valid

Populated, invalid

Not populated

Test plan

• Updated storybook and unit tests