Why this change?
Prior to this change, there is no description being displayed for
objects typed theme setting because we were rendering a button instead
of the components for the various setting types which will render the
setting's description.
What does this change do?
1. Introduce `SiteSettings::Description` compoment to centralise the HTML
being rendered across all settings component.
2. Renders the `SiteSettings::Description` component after the edit
button in `site_setting.hbs`.
