Fix various errors in german translation
Merge pull request #1311 from adrianlang/fixGerman

Fix various errors in german translation
Don't include `user_deleted` posts in emails.
Add `Discourse.Singleton` class mixin for creating singletons.
Convert `Discourse.User` to use `Discourse.Singleton`
Convert `Discourse.Site` and `Discourse.ScreenTrack` to use `Discourse.Singleton`
Fix delete user button on admin/user page so it's enabled at the right times, and update its title attribute to be more helpful
Updated docs for `Discourse.Singleton`
Suggested Topics tweak: Prefer topics in the same category