New work item modal on epic list page

What does this MR do and why?

Minimal attempt at Work items at the group level - create epic fro... (#429127 - closed)

Not as big as it looks! Diff is bigger due to moving some files around.

• Move most of create_work_item.vue from pages/ to components/
• Make previous page a shell over the form component
• Show a modal when clicking "New epic" on epics list page, that creates an Epic Work Item
• If you pass a workItemType to the form, don't show the workItemTypes select field
• After creating epic, show the epic

What is broken:

• Currently missing all the fields except title (coming soon !144705 (closed))

MR acceptance checklist

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Screenshots or screen recordings

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:namespace_level_work_items)
   Feature.enable(:work_items_mvc_2) # not strictly needed but helps be up to date

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

To test existing work_items form:

1. Go to work items new page (e.g. http://gdk.test:3000/gitlab-org/gitlab-test/-/work_items/new)
2. Enter title and select work item type
3. Create work item