Keep German translations in sync.
Merge pull request #925 from netzpirat/update-german-translations

Keep German translations in sync.
Support incomplete topic urls like /t/just-a-slug; fix error when using route /t/:topic_id/:post_number
Don't allow category slugs that are numbers
Move logic to reject slugs that are just numbers into the slug module
Refactored two class methods into scopes (to achieve a better 'rails way')
Also added some tests to the methods
Merge remote-tracking branch 'upstream/master'