Hook up create schedule mutation

What does this MR do and why?

Part of the refactor of pipeline schedules to Vue/GraphQL. Hooks up the create schedule mutation and refactors the form to work correctly for pipeline schedule creations. Currently editing schedules does not work yet, that will be part of the next MR.

Screenshots or screen recordings

How to set up and validate locally

1. Enable the feature flag Feature.enable(:pipeline_schedules_vue)
2. Visit Build > Pipeline schedules
3. Try creating a pipeline schedule with different values and test out

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.

Related to #385101 (closed)