Update translations for 1.4

[nemesifier]

New features introduced new strings. There are a bit of strings which don't have translations.
@eballarini could you guys help us out with the slovenian, friulan and german ones?

Take a look at the bottom of these files:

• https://github.com/openwisp/OpenWISP-User-Management-System/blob/master/config/locales/owums.fur.yml
• https://github.com/openwisp/OpenWISP-User-Management-System/blob/master/config/locales/owums.de.yml
• https://github.com/openwisp/OpenWISP-User-Management-System/blob/master/config/locales/owums.sl.yml

I would take care of the spanish translations.

[eballarini]

We are taking care of slovenian, friulan and german translations.