Build add pipeline subscription form

What does this MR do and why?

Part four: Refactoring pipeline subscriptions feature to Vue/Graphql. This refactor was broken down into multiple MRs for easy reviewing and faster iteration.

Goal of this MR

1. Add ability to add a pipeline subscription
2. Add test coverage

You can view rough outline of the following steps during this refactor here &11446

Part one: !139405 (merged)

Part two: !140812 (merged)

Part three: !141157 (merged)

Part four: You are here...

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. Enable feature flag in rails console pipeline_subscriptions_vue
2. Visit Project > Settings > CI/CD > Expand pipeline subscriptions
3. Try adding a new pipeline subscription

Related to #425293 (closed)