* fix: Enable access to block settings within UBE

The "Show more settings" menu item is no longer included in the block
toolbar after #46709 merged. Rather than relying upon that menu item,
this conditionally displays the sidebar toggle button whenever a block
is selected.

* fix: Disable white space stripping that breaks nested CSS selectors

CSS selectors rely upon a single white space between selectors to
represent an ancestor relationship. Globally removing white space in the
stylesheet breaks this functionality, as it transforms the selector to
target a single element with all the selectors.

The white space stripping should likely be replaced with a proper CSS
minification long term.

* fix: Hide block actions unrelated to editing a single block

Hide the entire "block settings" drop-down menu now that we no longer
rely upon it to access the "Show more settings" menu option that was
removed entirely.

* refactor: Relocate script toggling block settings visibility

This relates more to editor behavior than the post content.

* fix: Apply styles and script to editor canvas iframe

The editor canvas now relies upon an iframe. It is not possible to style
elements within an iframe from the parent context. This copies the
styles from the parent conext to the iframe.

Additionally, the logic selecting the first block also failed due to the
block not existing when it was invoked. This relocates that logic until
after the iframe is ready.

* fix: Expand conditional checks for partial DOM trees

On Android, there were times where the iframe was present, but the
nested window was not yet ready.

* refactor: Rename for brevity

* fix: Avoid React removing appended iframe styles

Append the styles to the `document` element, as React will remove the
mutation to the `head` element.

* docs: Add change log entry