Bug 40701: Add security warning when downloading a file

browser/components/downloads/content/allDownloadsView.js

.../components/downloads/content/contentAreaDownloadsView.js

...mponents/downloads/content/contentAreaDownloadsView.xhtml

browser/components/downloads/content/downloads.css

browser/components/downloads/content/downloads.js

...ser/components/downloads/content/downloadsPanel.inc.xhtml

browser/components/places/content/places.css

browser/components/places/content/places.js

browser/components/places/content/places.xhtml

browser/themes/shared/customizableui/panelUI-shared.css

browser/themes/shared/downloads/contentAreaDownloadsView.css

browser/themes/shared/downloads/downloads.inc.css