messages: Add English plural form of "Benutzer" and use it

German "Benutzer" is the same word for singular and plural, while the
English translation is either "User" or "Users", so we need two different
texts for it.

The menu entry must be "Users" instead of "User", and the caption of
the corresponding page must also be "Users". Fix this and translate
the comments for both code parts to English.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
messages: Fix English plural form "Dockets"

The German "Laufzettel" either translates to docket (singular) or
dockets (plural). For the menu and the corresponding caption, we need
the plural form in upper case "Dockets".

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Merge pull request #185 from stweil/fix-texts

Fix English texts in administration menu