GUI: Fixed display of sync status of groups

- Value in "synchronizationEnabled" attribute is String, so we can't
  just return value from attribute as JSNI boolean. It was wrong in
  conditions but correct on debug message.
- Added some null checks when processing RichGroups.
- Source format of JsonUtils.
Merge pull request #625 from zlamalp/sync

GUI: Fixed display of sync status of groups
RPC: Exclude PUT requests from pending requests

- Exclude PUT requests made by perun-engine from pending requests, since
  they contain a lot of data (listing / getData queries).
- TTL for pending requests lowered to 1 minute.
Merge pull request #626 from zlamalp/put

RPC: Exclude PUT requests from pending requests
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.