Bug 950171 - [settings] "Improve Firefox OS" title displayed as "Improve {{brandShortName}}"

Remove double substitution introduced in bug 944749, change string ID.
Merge pull request #14884 from flodolo/bug950171

Bug 950171 - [settings] "Improve Firefox OS" title displayed as "Improve {{brandShortName}}"
Bug 948314 - [Rocketbar] Marketplace App Search r=vingtetun
Merge pull request #14881 from KevinGrandon/bug_948314_rocketbar_marketplace_search

Bug 948314 - [Rocketbar] Marketplace App Search r=vingtetun