Add template for Alerts Service - frontend

What does this MR do?

Add Alerts section to Settings > Integrations page

See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/16117 for backend

Why does it do this

Part of https://gitlab.com/gitlab-org/gitlab-ee/issues/13203

The gross alternative was to add more haml field types, and inject just a Vue button. As shown in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/16029, this is not a good way

Testing locally

This is FE-only, some BE changes are being worked on separately.

To actually be able to view/navigate to the page, run either:

git revert 37a44c7e2be # Remove backend
git revert 6ad0d38f108 # enable JSON endpoint

Then enable the feature:

echo 'Feature.enable(:generic_alert_endpoint)' | bundle exec rails c

Then go to Settings > Integrations > Alerts endpoint

Screenshots

Caveats

1. The green dot next to 'Alerts endpoint' doesn't update live. I don't think this is a big deal

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content