GUI: support for observer role

 - allow to display vos/members tab
 - vo and members are clickable for new role
 - added support in session

TODO: groups tabs, content of all tabs, where vo_admin or vo_observer.
GUI: more support for VoObserver role

- VoObserver can now browse all tabs like vo admin, but all buttons
  and table checkboxes should be disabled.
- Tabs for add/assign/edit are not accessible for new role.
- Added new method to JsonCallbackEvents which can be used to pass data
  (results) from one callback to another.
- Added some custom messages for empty tables to vo/group management.
- Code format and cleanup.
GUI: more custom texts on empty tables

- Added more custom texts displayed on empty tables.
- Source format.
GUI: VO management usability fixes

- Custom texts in Vo tables now offer different message, when
  filtering on table is active. Because there is difference
  between "Vo has no members" and "No member matching 'sth' found.".

- Fixed styling of Application Form Settings table, so it now
  looks like other tables used in GUI. Display message when
  form exists but has no form items. Also form items which
  contain tables no longer display unnecessary border.