Merge pull request #916 from Supermathie/sysadmin-docs

documentation: merge & adapt suggestions from baus
Add email to German share link description help.
Improve German translations.

This tries to improve the German translations in its context, making
them fit better in the layout, unifying different terms, fixing typos,
try to improve the readability and add some missing translations.
Merge pull request #917 from netzpirat/improve-german-translations

Improve german translations
FIX: Categories pill wasn't always being highlighted properly.
FIX: Hitting ESC while autocomplete is open shouldn't close the composer.
Show excerpts on Categories page
url for uncategorized category topic list is always /category/uncategorized
Display details about the search context in the placeholder
Set Sidekiq log level to WARN
Set SeedFu to quiet mode. No talking please.
Set UTF-8 charset for plain text email part
Refactor: Less reliance on views for logic for topic list
JSHINT: extra comma
there is a scope in User model but not used in staff_constraint
Add warning on admin dashboard if production env is configured to send email through gmail
Trivial JS clean up
Use `{{aceEditor}}` helper to display the ace editor
Refactor: move category slug helper to Category model
Refactor: Adds `Discourse.Utilities.registerViewHelper` to register view helpers