Add confirmation modal to Deploy keys delete button

What does this MR do and why?

Part of #344927 (closed)

This MR adds a confirmation modal to the admin deploy keys delete button.

Screenshots or screen recordings

Screen_Recording_2021-11-23_at_2.42.29_PM

How to set up and validate locally

1. Fire up the Rails console
   • bin/rails console
2. Create some deploy keys

10.times do
    project = Project.order("RANDOM()").first
    deploy_key = FactoryBot.create(:deploy_key, public: true)
    FactoryBot.create(:deploy_keys_project, :write_access, deploy_key: deploy_key, project: project)
end

1. Enable the admin_deploy_keys_vue feature flag
   • Feature.enable(:admin_deploy_keys_vue)
2. Sign in as an Admin and navigate to /admin/deploy_keys
3. Click delete button for one of the deploy keys

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.