Docs: custom authentication now more obvious

I was having a bit of difficulty figuring out where exactly to implement my own authentication method, until I found http://dan.doezema.com/2012/02/how-to-implement-a-single-user-model-with-rails-activeadmin-and-devise/ and realized I should be implementing them in ApplicationController.

Hoping this helps fellow ActiveAdmin newbies.
change locale key order for better comparing to en version
add missing locale key de.active_admin.comments.author_missing
add missing locale key de.active_admin.access_denied
Merge pull request #3125 from timoschilling/de_locals

add some de locals
Merge pull request #3119 from AlexeyMK/master

Docs: custom authentication now more obvious
move collection decorator wrapping to its own class

There’s no functional change here; I just think it’s easier to
understand this way.