New work item modal on epic list page

What does this MR do and why?

• Add the create work item form to the epic list page in a modal
• if you pass a workItemType to the form, don't show the workItemTypes select field
• After creating epic, show the epic in a modal
• When closing the modal return to the list page

What is broken:

• You can only open the modal once
• Currently missing all the fields except title (coming soon)
• work item URL is wrong on epics page (has work item iid in place of epic iid)

MR acceptance checklist

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Screenshots or screen recordings

wi-epic

How to set up and validate locally

1. In rails console enable the experiment fully

   Feature.enable(:namespace_level_work_items)
   Feature.enable(:work_items_mvc_2)

2. Visit Epics page for a group
3. Click the New Epic button
4. Create an Epic