Add workflow to notify slack of milestone changes made by those external to team

[richard-cox]

Summary

Add a workflow to send a slack message if an issue milestone was changed from someone outside of rancher/ui

Technical notes summary

• If the user is in rancher/ui then the request to check it should succeed
• Check if the person who triggers the workflow is a member of rancher/ui
  • We can expand this to people outside of the team if required
• If the request fails the a notification is sent to a new rancher Rancher UI - Milestone Changed workflow
  • Currently sends me a DM, i'll change this once everything is working fine
  • Added a repo secret SLACK_WORKFLOW_MILESTONE_CHANGED_URL

Areas or cases that should be tested

• I've tested this in my fork
  • The check for membership always fails, i need to check it running from within this repo. have a feeling it won't work
  • The slack hook succesfully triggers the workflow and i get a DM with the correct values

TODO after merge

• Validate slack message is sent correctly given team member or not
• Change workflow to send a message to team channel instead of a DM to me

Checklist

• The PR is linked to an issue and the linked issue has a Milestone, or no issue is needed
• The PR has a Milestone
• The PR template has been filled out
• The PR has been self reviewed
• The PR has a reviewer assigned
• The PR has automated tests or clear instructions for manual tests and the linked issue has appropriate QA labels, or tests are not needed
• The PR has reviewed with UX and tested in light and dark mode, or there are no UX changes
• The PR has been reviewed in terms of Accessibility

[nwmac]

LGTM

[richard-cox]

TODO:

• Validate slack message is sent correctly given team member or not - FAIL always 40s, always sends DM
• Change workflow to send a message to team channel instead of a DM to me