code monitors: add "send test message" for slack and webhook actions

Created by: limitedmage

Part of #27163 (closed)

• Can now send test messages for Slack and webhook actions
• Refactored Email action to use the same architecture for test emails as Slack/webhooks (Apollo mutations)
• Wrote unit tests for all three action types

Test plan

• Extensive unit tests
• Manual tests using http://httpstat.us/ to simulate errors and https://webhooktest.com/ to test webhooks

Screenshots