User controls UI tweaks

Some tweaks arising from UX review of the gnome-initial-setup parental controls support, from Allan Day:

• Those rows are well wide.
• The rows should have dividing lines between them.
• Having controls labelled "allow" in a section headed "restrictions" is a bit mind-bending. If possible all the controls should be consistent in being restrictive or permissive. If the section is called "restrictions", each switch should be restrictive when it is on and permissive when it is off.
• If "All Ages" is a dropdown/popover thing, it ought to have a disclosure triangle (pan-down-symbolic) (like how a combobox looks).
• The triangle on the "Restrict Applications" row should use go-next-symbolic rather than pan-end-symbolic. Ha!
• Row description labels are sentences and should therefore have full stops.