This will allow consumers to inject it using `siteSettings: service()` in preparation for the removal of implicit injections in Ember 4.0. `site-settings:main` is still available and will print a deprecation notice.
Centralizes calculations in a helper under the site header component.
This also reverts a small CSS change to the composer: since ac79c5ef,
the composer height was not including the grippie, which means that the
composer height was off by 11 pixels, and the topic progress widget was
sometimes being displayed cut off by 11 pixels.
