FIX: Support J/K bindings on German Keyboards

J/K are not supported on German keyboards, shift+j and shift+k have been added to remedy this.
https://meta.discourse.org/t/keyboard-shortcuts-for-next-previous-section-do-not-work-with-german-keyboard-layout/18902
Change SHIFT+J and SHIFT+K to move up/down sections instead of posts.
FIX: Remove the old keyboard bindings for next/prev section, update client.en.yml to reflect the new bindings
Merge pull request #2725 from cpradio/german-keyboard-support

FIX: Support J/K bindings on German Keyboards