Use work item form for creating objectives

What does this MR do and why?

Use work item modal for creating new Objectives - #441684 (closed)

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

With okrs_mvc feature flag enabled:

Before	After

With okrs_mvc feature flag disabled: (no change)

How to set up and validate locally

1. In rails console: Feature.enable(:okrs_mvc)
2. Visit issues list for a project
3. Click the split button dropdown on 'New issue' and click 'New objective'
4. Fill out and submit the form (only title is required)