Merge pull request #14224 from flodolo/separate_header

Bug 944749 - [Settings][l12y] Use different labels for menu items and headers, r=kaze
(cherry picked from commit 588a3e02c4ace3b3341ba1f6bb7274120b53b2b3)

Removed change to apps/settings/elements/improve_browser_os.html which is going to be fixed by Bug 950171 (originally follow-up of this bug)

Conflicts:
	apps/settings/elements/downloads.html
	apps/settings/locales/settings.en-US.properties

(cherry picked from commit d256a30ef3fbdf8d70cfff2c12926df2ab9bf903)