Move portions of some post-merge widgets into the ready to merge widget

What does this MR do and why?

For #344835 (closed).

In !71721 (merged), the ready to merge (pre-merge) UI was made always visible.

Here, we make some adjustments so that:

• When the feature flag is on and
• certain components are being displayed

we split up the contents of those four components and re-render the components as children of the outer Ready To Merge widget with a special flag that toggles which part of the component to display.

Screenshots or screen recordings

	Before	After
Merging
Merged
Closed
Auto-merge Enabled

How to set up and validate locally

1. Enable the restructured_mr_widget feature flag
2. On lines 582 & 585 of app/assets/javascripts/vue_merge_request_widget/mr_widget_options.vue, change each instance of componentName to one of:
   • mr-widget-merging
   • mr-widget-merged
   • mr-widget-closed
   • mr-widget-auto-merge-enabled
3. View an MR to review the widget

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.