messages: Use English plural form "Users"

The database statistics also need the plural form "Users".

Signed-off-by: Stefan Weil <sw@weilnetz.de>
messages: Fix duplicate article "the the"

Fix the duplicate "the the" (and one more typo) also in code comments
in other files.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Merge pull request #191 from stweil/fix-texts

Small fixes for comments, English messages and user interface