What?

Fix the position of the URLPopover used in the MediaPlaceholder component by explicitly providing a ref.

Why?

I noticed that the position of the URLPopover in the MediaPlaceholder component was incorrect. After doing a git bisect it appears that the issue has been present since #46212 when the post editor was updated to use an iframe. The issue was most noticeable with the list view open.

I'm not 100% sure the exact cause, but my suspicion is that when using an implicit parent approach for the Popover component, it doesn't play nicely when it has to cross iframe boundaries sometimes. Without going too much further into it, this fix of explicitly passing down a ref to the expected container appears to work.

How?

• Move the innards of renderUrlSelectionUI over to a separate component, add a useState to grab a ref for the input container. Pass the ref to the URLPopover.
• I preserved the renderUrlSelectionUI function as-is to try to keep this diff as small as possible, since that function call is used in several places. It seemed a bit neater to preserve that function call than replace it with <URLSelectionUI /> directly.

Testing Instructions

1. Test with a blocks theme, e.g. TT3
2. With the List View open in a post or page, add an Image block
3. Click on the Insert from URL button
4. Note that on trunk the position of the popover will be incorrect. With this PR applied, the popover should be positioned beneath the input container

Screenshots or screencast

Before	After