Administration: Make some adjustments to `WP_Screen::render_view_mode()`:

* Restore the `$mode` global for backward compatibility.
* Remove redundant check, as `$mode` is already set at this point, and already defaults to `list` via `get_user_setting()`'s second argument.
* Use sentence case for "View mode" and "Extended view" labels.

Follow-up to [48398].

See #49715.
Built from https://develop.svn.wordpress.org/trunk@48423


git-svn-id: https://core.svn.wordpress.org/trunk@48192 1a063a9b-81f0-0310-95a4-ce76da25c4cd