Add MWC home & tabzilla promos. Bug 958294.
Bug 970870 :  Restrict Tabzilla MWC promo to MWC locales
* Tabzilla promo activated via a promo_mwc_2014 tag in tabzilla.lang
* Locales without the promo get the current Firefox OS promo (which is widely localized)
* Promo strings extracted from mwc2014_promos.lang for both the tabzilla promo and the home page tile, to restrict localization to 16 selected locales