GUI: fixed display of sync of interval on groups
Merge pull request #627 from zlamalp/fix

GUI: fixed display of sync of interval on groups
REGISTRAR,GUI: allow to change native language

- Allow to set native language for notifications and registration
  in perun.properties and perun-web-gui.properties files.
GUI: Support for custom native language

- Show also unused (but stored) languages in form item texts and
  notifications to support native lang transition.
- On buttons, removed Czech flag when switching language, display
  native language name.
- Fixed calls to get/set form items and mail to send all possible languages.
- Properly handle native language when displaying form items.
- Offer custom native language as option on user detail - preferred language.