Resolve "Support alerts from external Prometheus servers" ~frontend

What does this MR do?

Add section to get Notify URL and Auth key, and button to reset key.

Go to Settings > Integrations > Prometheus to see the new section and start creating the keys.

key2

CE Backport (1 line): https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24818

What are the relevant issue numbers?

Based on https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9238 Based on https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9334

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary - https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9334
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer

Closes #9258 (closed)