Implement UX designs for failed webhooks: 400 error range

This relates to work in &6004 to improve the reliability of webhooks.

To do so, we need to inform users when their webhooks have failed, for errors in the 400 range.

Designs

See: #331407[Scenario_1.png]

To Do

• Add danger alert (dismissible) to project/group/admin webook edit pages
  • app/views/projects/hooks/edit.html.haml
  • ee/app/views/groups/hooks/edit.html.haml
  • app/views/admin/hooks/edit.html.haml

  Webhook failed to connect
  
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• Add danger badge to app/views/shared/web_hooks/_hook.html.haml

  Failed to connect