Implement safe HTML translations

[roblillack]

As part of #1 we disabled implicitly marking translation output as safe HTML without escaping (WTH?). Some users might want to use safe HTML translations. Let's implement this in a way comparable to Ruby on Rails:

• The translation key has to end in _html or .html if you want to use HTML as part of the translation string
• The translation function shall always escape parameters that are not already safe HTML before interpolating them into the translation string.