Bug 883813 - [SMS][MMS] Placeholder string "Message" is not localizable r=julienw

Use HTML5 "data-" attribute for the localization value because setting
arbitrary elements is explicitly *not* supported by Gaia's L10n library.
(cherry picked from commit 1c3c806721b973b03d083cd36f9ca338cedeb34f)

Conflicts:
	apps/sms/locales/sms.ar.properties
	apps/sms/locales/sms.fr.properties